CHAPTER I – INTRODUCTION

To derive strength equal to the daily task; to experience the
advantages of health and avoid the pain, inconvenience, and danger of
disease; to live out contentedly and usefully the natural span of life:
these are problems that concern all people. They are, however, but
different phases of one great problem—the problem of properly managing
or caring for the body. To supply knowledge necessary to the solution of
this problem is the chief reason why the body is studied in our public
schools.

Divisions of the Subject.—The body
is studied from three standpoints: structure, use of parts, and care or
management. This causes the main subject to be considered under three
heads, known as anatomy, physiology, and hygiene.

Anatomy treats of the construction
of the body—the parts which compose it, what they are like, and where
located. Its main divisions are known as gross anatomy and histology.
Gross anatomy treats of the larger
structures of the body, while histology treats of the minute structures of which these
are composed—parts too small to be seen with the naked eye and which
have to be studied with the aid of the microscope.

[pg 002]Physiology treats of the function, or use, of the different
parts of the body—the work which the parts do and how they do it—and of
their relations to one another and to the body as a whole.

Hygiene treats of the proper care
or management of the body. In a somewhat narrower sense it treats of the
“laws of health.” Hygiene is said to be personal, when applied by the individual to his own body;
domestic, when applied to a small
group of people, as the family; and public, or general, when applied to the community as a whole or to the
race.

The General Aim of Hygiene.—There
are many so-called laws of health, and for these laws it is essential in
the management of the body to find a common basis. This basic law,
suggested by the nature of the body and conditions that affect its
well-being, may be termed the Law of
Harmony: The mode of living must harmonize with the plan of the
body. To live properly one must supply the conditions which his
body, on account of its nature and plan, requires. On the other hand, he
must avoid those things and conditions which are injurious, i.e., out of harmony with the body plan.
To secure these results, it is necessary to determine what is and what
is not in harmony with the plan of the body, and to find the means of
applying this knowledge to the everyday problems of living. Such is the
general aim of hygiene. Stated in other words: Hygiene has for its
general aim the bringing about of an essential harmony between the body
and the things and conditions that affect it.1

[pg 003]Relation of Anatomy and Physiology to the Study of
Hygiene.—If the chief object in studying the body is that of
learning how to manage or care for it, and hygiene supplies this
information, why must we also study anatomy and physiology? The answer
to this question has already been in part suggested. In order to
determine what things and conditions are in harmony with the plan of the
body, we must know what that plan is. This knowledge is obtained through
a study of anatomy and physiology. The knowledge gained through these
subjects also renders the study of hygiene more interesting and
valuable. One is enabled to see why
and how obedience to hygienic laws
benefits, and disobedience to them injures, the body. This causes the
teachings of hygiene to be taken more seriously and renders them more
practical. In short, anatomy and physiology supply a necessary basis for
the study of hygiene.

Advantages of Properly Managing the
Body.—One result following the mismanagement of the body is loss of
health. But attending the loss of health are other results which are
equally serious and far-reaching. Without good health, people fail to
accomplish their aims and ambitions in life; they miss the joy of
living; they lose their ability to work and become burdens on their
friends or society. The proper management of the body means health, and
it also means the capacity for work and for enjoyment. Not only should
one seek to preserve his health from day to day, but he should so manage
his body as to use his powers to the best advantage and prolong as far
as possible the period during which he may be a capable and useful
citizen.

CHAPTER II – GENERAL VIEW OF THE BODY

External Divisions.—Examined from
the outside, the body presents certain parts, or divisions, familiar to
all. The main, or central, portion is known as the trunk, and to this are attached the head, the upper
extremities, and the lower
extremities. These in turn present smaller divisions which are also
familiar. The upper part of the trunk is known as the thorax, or chest, and the lower part as
the abdomen. The portions of the
trunk to which the arms are attached are the shoulders, and those to which the legs are joined are the
hips, while the central rear portion
between the neck and the hips is the back. The fingers, the hand, the wrist, the forearm, the
elbow, and the upper arm are the main divisions of each of the upper
extremities. The toes, the foot, the ankle, the lower leg, the knee, and
the thigh are the chief divisions of each of the lower extremities. The
head, which is joined to the trunk by the neck, has such interesting
parts as the eyes, the ears, the nose, the jaws, the cheeks, and the
mouth. The entire body is inclosed in a double covering, called the skin, which protects it in various
ways.

The Tissues.—After examining the
external features of the body, we naturally inquire about its internal
structures. These are not so easily investigated, and much which is of
interest to advanced students must be omitted from an elementary course.
We may, however, as a first step in this study, determine what kinds of
materials enter into [pg 005]the
construction of the body. For this purpose the body of some small animal
should be dissected and studied. (See observation at close of chapter.)
The different materials found by such a dissection correspond closely to
the substances, called tissues, which
make up the human body. The main tissues of the body, as ordinarily
named, are the muscular tissue, the
osseous tissue, the connective tissue, the nervous tissue, the adipose tissue, the cartilaginous tissue, and the epithelial and glandular
tissue. Most of these present different varieties, making all together
some fifteen different kinds of tissues that enter into the construction
of the body.2

General Purposes of the
Tissues.—The tissues, first of all, form the body. As a house is constructed of wood, stone,
plaster, iron, and other building materials, so is the body made up of
its various tissues. For this reason the tissues have been called the
building materials of the body.

In addition to forming the body, the tissues supply the means through
which its work is carried on. They are thus the working materials of the body. In serving this purpose the
tissues play an active rôle. All of them must perform the activities of
growth and repair, and certain ones (the so-called active tissues) must
do work which benefits the body as a whole.

Purposes of the Different
Tissues.—In the construction of the body and also in the work which
it carries on, the different tissues are made to serve different
purposes. The osseous tissue is the chief substance in the bony
framework, or skeleton, while the muscular tissue produces the different
movements of the body. The connective[pg 006] tissue, which is everywhere abundant, serves the general
purpose of connecting the different parts together. Cartilaginous tissue
forms smooth coverings over the ends of the bones and, in addition to
this, supplies the necessary stiffness in organs like the larynx and the
ear. The nervous tissue controls the body and brings it into proper
relations with its surroundings, while the epithelial tissue (found upon
the body surfaces and in the glands) supplies it with protective
coverings and secretes liquids. The adipose tissue (fat) prevents the
too rapid escape of heat from the body, supplies it with nourishment in
time of need, and forms soft pads for delicate organs like the
eyeball.

Properties of the Tissues.—If we
inquire how the tissues are able to serve such widely different
purposes, we find this answer. The tissues differ from one another both
in composition and in structure and, on this account, differ in their
properties.3 Their different properties enable them
to serve different purposes in the body. Somewhat as glass is adapted by
its transparency, hardness, and toughness to the use made of it in
windows, the special properties of the tissues adapt them to the kinds
of service which they perform. Properties that adapt tissues to their
work in the body are called essential
properties. The most important of these essential properties are as
follows:

1. Of osseous tissue, hardness, stiffness, and toughness. 2. Of
muscular tissue, contractility and irritability. 3. Of nervous tissue,
irritability and conductivity. 4. Of cartilaginous tissue, stiffness and
elasticity. 5. Of connective tissue, toughness and pliability. 6. Of
epithelial tissue, ability to resist the action of external forces and
power to secrete.

[pg 007]

Fig. 1—Hand and forearm, showing the grouping of muscular and
connective tissues in the organ for grasping.

Tissue Groups.—In the construction
of the body the tissues are grouped together to form its various
divisions or parts. A group of tissues which serves some special purpose
is known as an organ. The hand, for
example, is an organ for grasping (Fig. 1). While the different organs
of the body do not always contain the same tissues, and never contain
them in the same proportions, they do contain such tissues as their work
requires and these have a special arrangement—one adapted to the work
which the organs perform.

In addition to forming the organs, the tissues are also grouped in
such a manner as to provide supports for organs and to form cavities in
which organs are placed. The various cavities of the body are of
particular interest and importance. The three largest ones are the cranial cavity, containing the brain; the
thoracic cavity, containing the heart
and the lungs; and the abdominal
cavity, containing the stomach, the liver, the intestines, and other
important organs (Fig. 2). Smaller cavities serving different purposes
are also found.

[pg 008]

Fig. 2—Diagram of a lengthwise section of the body to show its
large cavities and the organs which they contain.

Organs and Systems.—The work of the
body is carried on by its various organs. Many, in fact the majority, of
these organs serve more than one purpose. The tongue[pg 009]
is used in talking, in
masticating the food, and in swallowing. The nose serves at least three
distinct purposes. The mouth, the arms, the hands, the feet, the legs,
the liver, the lungs, and the stomach are also organs that serve more
than one purpose. This introduces the principle of economy into the
construction of the body and diminishes the number of organs that would
otherwise be required.

The various organs also combine
with one another in carrying on the work of the body. An illustration of
this is seen in the digestion of the food—a process which requires the
combined action of the mouth, stomach, liver, intestines, and other
organs. A number of organs working together for the same purpose form a
system. The chief systems of the body
are the digestive system, the circulatory system, the respiratory
system, the muscular system, and the nervous system.

The Organ and its Work.—A most
interesting question relating to the work of the organ is this: Does the
organ work for its own benefit or for the benefit of the body as a
whole? Does the hand, for example, grasp for itself or in order that the
entire body may come into possession? Only slight study is sufficient to
reveal the fact that each organ performs a work which benefits the body
as a whole. In other words, just as the organ itself is a part of the
body, the work which it does is a part of the necessary work which the
body has to do.

But in working for the general good, or for the body as a whole, each
organ becomes a sharer in the benefits of the work done by every other
organ. While the hand receives only a little of the nourishment
contained in the food which it places in the mouth or of the heat from,
fuel which it places on the fire, it is aided and supported by the work
of all the other organs of the body—eyes, [pg 010] feet, brain, heart, etc. The hand does not and cannot work
independently of the other organs. It is one of the partners in a very
close combination where, by doing a particular work, it, shares in the
profits of all. What is true of the hand is true of every other organ of
the body.

An Organization.—The relations
which the different organs sustain to each other and to the body as a
whole suggest the possibility of classifying the body as an
organization. This term is broadly applied to a variety of combinations.
An organization is properly defined as any
group of individuals which, in working together for a common purpose,
practices the division of labor. This definition will be better
understood by considering a few familiar examples.

A baseball team is an organization. The team is made up of individual
players. These work together for the common purpose of winning games.
They practice the division of labor in that the different players do
different things—one catching, another pitching, and so on. A
manufacturing establishment which employs several workmen may also be an
organization. The article manufactured provides the common purpose
toward which all strive; and, in the assignment of different kinds of
work to the individual workmen, the principle of division of labor is
carried out. For the same reason a school, a railway system, an army,
and a political party are organizations.

An organization of a lower order of individuals than these human
organizations is to be found in a hive of bees. This is made up of the
individual bees, and these, in carrying on the general work of the hive,
are known to practice the division of labor.

Is the Body an Organization?—If the
body is an organization, it must fulfill the conditions of the
definition. It[pg 011] must be made up of
separate or individual parts. These must work together for the same
general purpose, and, in the accomplishment of this purpose, must
practice the division of labor. That the body practices the division of
labor is seen in the related work of the different organs. That it is
made up of minute, but individual, parts will be shown in the chapter
following. That it carries on a general
work which is accomplished through the combined action of its
individual parts is revealed through an extended study of its various
activities. The body is an
organization. Moreover, it is one of the most complex and, at the
same time, most perfect of the organizations of which we have
knowledge.

Summary.—Viewed from the outside,
the body is seen to be made up of divisions which are more or less
familiar. Viewed internally, it is found to consist of different kinds
of materials, called tissues. The tissues are adapted, by their
properties, to different purposes both in the construction of the body
and in carrying on its work. The working parts of the body are called
organs and these in their work combine to form systems. The entire body,
on account of the method of its construction and the character of its
work, may be classed as an organization.

Exercises.—1. Name and locate the
chief external divisions of the body.

2. What tissues may be found by dissecting the leg of a chicken?

3. Name the most important properties and the most important uses of
muscular tissue, osseous tissue, and connective tissue.

4. Define an organ. Define a system. Name examples of each.

5. Name the chief cavities of the body and the organs which they
contain.

6. What tissues are present in the hand? How does each of these aid
in the work of the hand?

[pg 012]7. Define an organization. Show
that a railway system, an army, and a school are organizations.

8. What is meant by the phrase “division of labor”? In what manner is
the division of labor practiced in a shoe or watch factory? What are the
advantages?

9. What are the proofs that the body is an organization?

CHAPTER III – THE BODY ORGANIZATION

What is the nature of the body organization? What are the individual
parts, or units, that make it up? What general work do these carry on
and upon what basis do they practice the division of labor? The answers
to these questions will suggest the main problems in the study of the
body.

Fig. 3—Diagram
showing the relation of the cells and the intercellular material. C. Cells. I. Intercellular material.

Complex Nature of the Tissues.—To
the unaided eye the tissues have the appearance of simple structures.
The microscope, however, shows just the reverse to be true. When any one
of the tissues is suitably prepared and carefully examined with this
instrument, at least two classes of materials can be made out. One of
these consists of minute particles, called cells; the other is a substance lying between the cells,
known as the intercellular material
(Fig. 3). The cells and the intercellular material, though varying in
their relative proportions, are present in all the tissues.

The Body a Cell Group.—The
biologist has found that the bodies of all living things, plants as well
as animals, consist either of single cells or of groups of cells. The
single cells live independently of one another, but the cells that form
groups are attached to, and are more or less dependent upon, one
another. In the first condition are [pg 014]
found the very lowest forms of life. In the second, life reaches its
greatest development. The body of man, which represents the highest type
of life, is recognized as a group of cells. In this group each cell is
usually separate and distinct from the others, but is attached to them,
and is held in place by the intercellular material.

Protoplasm, the Cell Substance.—The
cell is properly regarded as an organized bit of a peculiar material, called protoplasm. This is a semi-liquid and
somewhat granular substance which resembles in appearance the white of a
raw egg. Its true nature and composition are unknown, because any
attempt to analyze it kills it, and dead protoplasm is essentially
different from living protoplasm. It is known, however, to be a highly
complex substance and to undergo chemical change readily. It appears to
be the only kind of matter with which life is ever associated, and for
this reason protoplasm is called the physical basis of life. Its organization into separate
bits, or cells, is necessary to the life activities that take place
within it.

Structure of the Cell.—Though all
portions of the cell are formed from the protoplasm, this essential
substance differs both in structure and in function at different places
in the cell. For this reason the cell is looked upon as a complex body
having several distinct parts. At or near the center is a clear, rounded
body, called the nucleus. This plays
some part in the nourishment of the cell and also in the formation of
new cells. If it be absent, as is sometimes the case, the cell is
short-lived and unable to reproduce itself. The variety of protoplasm
contained in the nucleus is called the nucleoplasm.

Fig. 4—Diagram of a
typical cell (after Wilson). 1. Main body. 2. Nucleus. 3.
Attraction sphere. 4. Food particles and waste. 5. Cell-wall. 6. Masses
of active material found in certain cells, called plastids.

Surrounding the nucleus is the main
body of the cell, sometimes referred to as the “protoplasm.” Since
the[pg 015] protoplasm forms all parts of
the cell, this substance is more properly called the cytoplasm, or cell plasm. Surrounding and
inclosing the cytoplasm, in many cells, is a thin outer layer, or
membrane, which affords more or less protection to the contents of the
cell. This is usually referred to as the cell-wall. A fourth part of the cell is also described,
being called the attraction sphere.
This is a small body lying near the nucleus and coöperating with that
body in the formation of new cells. Food particles, wastes, and other
substances may also be present in the cytoplasm. The parts of a typical
cell are shown in Fig. 4.

Importance of the Cells.—The cells
must be regarded as the living, working parts of the body. They are the
active agents in all of the tissues, enabling them to serve their
various purposes. Working through the tissues, they build up the body
and carry on its different activities. They are recognized on this
account as the units of structure and of
function, and are the “individuals” in the body organization. Among
the most important and interesting of the activities of the cells are
those by which they build up the body, or cause it to grow.

[pg 016]How the Cells enable the Body to
Grow.—Every cell is able to take new material into itself and to
add this to the protoplasm. This tends to increase the amount of the
protoplasm, thereby causing the cells to increase in size. A general
increase in the size of the cells has the effect of increasing the size
of the entire body, and this is one way by which they cause it to grow.
There is, however, a fixed limit, varying with different cells, to the
size which they attain, and this is quite low. (The largest cells are
scarcely visible to the naked eye.) Any marked increase in the size of
the body must, therefore, be brought about by other means. Such a means
is found in the formation of new cells, or cell reproduction. The new cells are always formed by and from the old cells, the essential process being known as
cell-division.

Fig. 5—Steps in
cell-division (after Wilson). Note that the process begins with the
division of the attraction sphere, then involves the nucleus, and
finally separates the main body.

Cell-Division.—By dividing, a
single cell will, on attaining its growth, separate into two or more new
cells. The process is quite complex and is imperfectly understood. It is
known, however, that the act of separation is preceded by a series of
changes in which the attraction sphere[pg 017] and the nucleus actively
participate, and that, as a result of these changes, the contents of the
old cell are rearranged to form the new cells. Some of the different
stages in the process, as they have been studied under the microscope,
are indicated in Fig. 5.

Gradually, through the formation of new cells and by the growth of
these cells after they have been formed, the body attains its full size.
When growth is complete, cell reproduction is supposed to cease except
where the tissues are injured, as in the breaking of a bone, or where
cells, like those at the surface of the skin, are subject to wear. Then
new material continues to be added to the protoplasm throughout life,
but in amount only sufficient to replace that lost from the protoplasm
as waste.

Fig. 6—A tumbler partly filled with marbles covered
with water, suggesting the relations of the cells to the lymph.

Cell Surroundings.—All cells are
said to be aquatic. This means simply
that they require water for carrying on their various activities. The
cells, in order to live, must take in and give out materials, and water
is necessary to both processes. It is also an essential part of the
protoplasm. Deprived of water, cells become inactive and usually die.
Aquatic surroundings are provided for the cells of the body through a
liquid known as the lymph, which is
distributed throughout the intercellular material (Fig. 6). This
consists of water containing oxygen and food substances in solution.
Besides supplying these to the cells, the lymph also receives their
wastes. Through the lymph the necessary conditions for cell life are
provided in the body.

The General Work of Cells.—In
handling the materials[pg 018] derived from the lymph, the cells carry
on three well-defined processes, known as absorption, assimilation, and
excretion.

Absorption is the process of
taking water, food, and oxygen into the cells.

Assimilation is a complex process
which results in the addition of the absorbed materials to the
protoplasm. Through assimilation the protoplasm is built up or
renewed.

Excretion is the throwing off of
such waste materials as have been formed in the cells. These are passed
into the lymph and thence to the surface of the body.

Absorption, assimilation, excretion, and also reproduction are
performed by all classes of cells. They are, on this account, referred
to as the general work of cells.

The Special Work of Cells.—In
addition to the general work which all cells do in common, each class of
cells in the body is able to do some particular kind of work—a work
which the others cannot do or which they can do only to a limited
extent. This is spoken of as the special
work of cells. Examples of the special work of cells are found in
the production of motion by muscle cells and in the secretion of liquids
by gland cells. It may be noted that while the general work of cells
benefits them individually, their special work benefits the body as a
whole. Another example of the special work of cells is found in the

Fig. 7—Cartilage cells, surrounded by the intercellular material
which they have deposited.

Production of the Intercellular
Material.—Though most of the cells of the body deposit to a slight
extent this material, the greater part of it is produced by a single
class of cells found in bone, cartilage, and connective tissue.
Cartilage, bone, and connective tissue differ greatly from the other
tissues in the amount of intercellular material which they contain, the
difference being due to these cells.[pg 019]
In the connective tissue they deposit the fibrous material so
important in holding the different parts of the body together. In the
cartilage they produce the gristly substance which forms by far its
larger portion (Fig. 7). In the bones they deposit a material similar to
that in the cartilage, except that with it is mixed a mineral substance
which gives the bones their hardness and stiffness.4 The intercellular
material, in addition to connecting the cells, supplies to certain
tissues important properties, such as the elasticity of cartilage and
the stiffness of the bones.

Nature of the Body
Organization.—The division of labor carried on by the different
organs, as shown in the preceding chapter, is in reality carried on by
the cells that form the organs. To see that this is true we have only to
observe the relation of cells to tissues and of tissues to organs. The
cells form the tissues and the tissues form the organs. This arrangement
enables the special work of different kinds of cells to be combined in
the work of the organ as a whole. This is seen in the hand which, in
grasping, uses motion supplied by the muscle cells, a controlling
influence supplied by the nerve cells, a framework supplied by the bone
cells, and so on. The cells supply the basis for the body organization
and, properly speaking, the body is an
organization of cells5 (Recall the definition[pg 020] of an organization, page 10.) In
this organization there are to be observed:

1. A definite arrangement of the cells to form the tissues. A tissue
is a group of like cells.

2. A definite arrangement of the tissues in the organ. Each organ
contains the tissues needed for its work.

3. In several instances there is a definite arrangement of organs to
form systems.

4. The body as a whole is made up of organs and systems, together
with the structures necessary for their support and protection.

There now remains a further question for consideration. What is the
one supreme end, or purpose, toward which all the activities of the body
organization are directed? This purpose will naturally have some
relation to the maintenance, or preservation, of the cell group which we
call the body.

The Maintenance of Life.—The
preservation of any cell group in its natural condition, whether it be
plant or animal, is accomplished through keeping it alive. If life
ceases, the group quickly disintegrates and its elements become
scattered, a fact which is verified through everyday observation. Though
the nature of life is unknown, it may be looked upon as the organizer
and preserver of the protoplasm. But in preserving the protoplasm it
also preserves the entire cell group, or body. Life is thus the most
essential condition of the body. With life
all portions of the body are concerned, and toward its maintenance all
the activities of the body organization are directed.

The Nutrient Fluid in its Relations to
the Cells.—The maintenance of life within the cells requires, as we
have seen, that they be supplied with water, food, and oxygen, and that
they be relieved of such wastes as they form.[pg 021] This double purpose is accomplished through the agency of
an internal nutrient fluid, a portion of which has already been referred
to as the lymph. Not only does this fluid supply the means for keeping
the cells alive, but, through the cells, it is also the means of
preserving the life of the body as a whole.

The cells, however, rapidly exhaust the nutrient fluid. They take
from it food and oxygen and they put into it their wastes. To prevent
its becoming unfit for supplying their needs, food and oxygen must be
continually added to this fluid, and waste materials must be continually
removed. This is not an easy task. As a matter of fact, the preparation,
distribution, and purification of the nutrient fluid requires the direct
or indirect aid of practically all parts of the body. It supplies for
this reason a broad basis for the division of labor on the part of the
cells.

Relation of the Body to its
Environment.—While life is directly dependent upon the internal
nutrient fluid, it is indirectly dependent upon the physical
surroundings of the body. Herein lies the need of the external organs—the feet and legs for
moving about, the hands for handling things, the eyes for directing
movements, etc. That the great needs of the body are supplied from its
surroundings are facts of common experience. Food, shelter, air,
clothing, water, and the means of protection are external to the body
and form a part of its environment. In making the things about him
contribute to his needs, man encounters a problem which taxes all his
powers. Only by toil and hardship, “by the sweat of his brow,” has he
been able to wrest from his surroundings the means of his
sustenance.

The Main Physiological
Problems.—The study of the body is thus seen to resolve itself
naturally into the consideration of two main problems:

[pg 022]1. That of maintaining in the body a nutrient fluid for the
cells.

2. That of bringing the body into such
relations with its surroundings as will enable it to secure materials
for the nutrient fluid and satisfy its other needs.

The first problem is internal and
includes the so-called vital processes, known as digestion, circulation,
respiration, and excretion. The second problem is external, as it were, and includes the work of the external
organs—the organs of motion and of locomotion and the organs of special
sense. These problems are closely related, since they are the two
divisions of the one problem of maintaining life. Neither can be
considered independently of the other. In the chapter following is taken
up the first of these problems.

Summary.—The individual parts, or
units, that form the body organization are known as cells. These consist
of minute but definitely arranged portions of protoplasm and are held
together by the intercellular material. They build up the body and carry
on its different activities. The tissues are groups of like cells. By
certain general activities the cells maintain their existence in the
tissues and by the exercise of certain special activities they adapt the
tissues to their purposes in the body. The body, as a cell organization,
has its activities directed under normal conditions toward a single
purpose—that of maintaining life. In the accomplishment of this purpose
a nutrient fluid is provided for the cells and proper relations between
the body and its surroundings are established.

Exercises.—1. If a tissue be
compared to a brick wall, to what do the separate bricks correspond? To
what the mortar between the bricks?

2. Draw an outline of a typical cell, locating and naming the main
divisions.

3. How do the cells enable the body to grow? Describe the process of
cell-division.

[pg 023]4. How does the
general work of cells differ from their special work? Define absorption,
excretion, and assimilation as applied to the cells.

5. Compare the conditions surrounding a one-celled animal, living in
water, to the conditions surrounding the cells in the body.

6. What is meant by the term “environment”? How does man’s
environment differ from that of a fish?

7. What is the necessity for a nutrient fluid in the body?

8. Why is the maintenance of life necessarily the chief aim of all
the activities of the body?

9. State the two main problems in the study of the body.

CHAPTER IV – THE BLOOD

Two liquids of similar nature are found in the body, known as the
blood and the lymph. These are closely related in function and together
they form the nutrient fluid referred to in the preceding chapter. The
blood is the more familiar of the two liquids, and the one which can
best be considered at this time.

The Blood: where Found.—The blood
occupies and moves through a system of closed tubes, known as the blood
vessels. By means of these vessels the blood is made to circulate
through all parts of the body, but from them it does not escape under
normal conditions. Though provisions exist whereby liquid materials may
both enter and leave the blood stream, it is only when the blood vessels
are cut or broken that the blood, as blood, is able to escape from its
inclosures.

Physical Properties of the
Blood.—Experiments such as those described at the close of this
chapter reveal the more important physical properties of the blood. It
may be shown to be heavier and denser than water; to have a faint odor
and a slightly salty taste; to have a bright red color when it contains
oxygen and a dark red color when oxygen is absent; and to undergo, when
exposed to certain conditions, a change called coagulation. These
properties are all accounted for through the different materials that
enter into the formation of the blood.

Fig. 8—Blood corpuscles, highly magnified. A.
Red corpuscles as they appear in diluted blood. B. Arrangement of red corpuscles in rows between which are
white corpuscles, as may be seen in undiluted blood. C. Red corpuscles much enlarged to show
the form.

[pg 025]Composition of the Blood.—To the
naked eye the blood appears as a thick but simple liquid; but when
examined with a compound microscope, it is seen to be complex in nature,
consisting of at least two distinct portions. One of these is a clear,
transparent liquid; while the other is made up of many small, round
bodies that float in the liquid. The liquid portion of the blood is
called the plasma; the small bodies
are known as corpuscles. Two
varieties of corpuscles are described—the red corpuscles and the white corpuscles (Fig. 8). Other round particles, smaller
than the corpuscles, may also be seen under favorable conditions. These
latter are known as blood
platelets.

Red Corpuscles.—The red corpuscles
are classed as cells, although, as found in the blood of man and the
other mammals (Fig. 9), they have no nuclei.6 Each one consists of a little mass of protoplasm,
called the stroma, which contains a
substance having a red color, known as hemoglobin. The shape of the red corpuscle is that of a
circular disk with concave sides. It has a width of about 1/3200 of an
inch (7.9 microns7) and a thickness of[pg 026] about 1/13000 of an inch (1.9 microns). The red
corpuscles are exceedingly numerous, there being as many as five
millions in a small drop (one cubic millimeter) of healthy blood. But
the number varies somewhat and is greatly diminished during certain
forms of disease.

Fig. 9—Red corpuscles
from various animals. Those from mammals are without nuclei, while
those from birds and cold-blooded animals have nuclei.

It is the function of the red
corpuscles to serve as oxygen
carriers for the cells. They take up oxygen at the lungs and
release it at the cells in the different tissues.8 The
performance of this function depends upon the hemoglobin.

Hemoglobin.—This substance has the
remarkable property of forming, under certain conditions, a weak
chemical union with oxygen and, when the conditions are reversed, of
separating from it. It forms[pg 027] about nine tenths of the solid
matter of the red corpuscles and to it is due the colors of the blood.
When united with the oxygen it forms a compound, called oxyhemoglobin, which has a bright red
color; the hemoglobin alone has a dark red color. These colors are the
same as those of the blood as it takes on and gives off oxygen. The
stroma, which forms only about one tenth of the solid matter of the
corpuscles, serves as a contrivance for holding the hemoglobin. The
conditions which cause the hemoglobin to unite with oxygen in the lungs
and to separate from it in the tissues, will be considered later
(Chapter VIII).

Disappearance and Origin of Red
Corpuscles.—The red corpuscles, being cells without nuclei, are
necessarily short-lived. It has been estimated that during a period of
one to two months, all the red corpuscles in the body at a given time
will have disappeared and their places taken by new ones. The origin of
new corpuscles, however, and the manner of ridding the blood of old ones
are problems that are not as yet fully solved. The removal of the
products of broken down corpuscles is supposed to take place both in the
liver and in the spleen.9

Regarding the origin of the red corpuscles, the evidence now seems
conclusive that large numbers of them are formed in the red marrow of
the bones. The red marrow is located in what is known as the spongy
substance of the bones (Chapter XIV) and consists, to a large extent, of
cells somewhat like the red corpuscles, but differing from them in
having nuclei. These appear to be constantly in a state of reproduction.
The blood, flowing through the minute cavities containing these cells,
carries those that have been loosened out into the blood stream. Nuclei
appear in the red corpuscles at the time of their formation, but these
quickly separate and, according to some authorities, form the blood
platelets.

White Corpuscles.—The white
corpuscles, or leucocytes, are cells
of a general spherical shape, each containing one, two, or more nuclei.
They are much less numerous than the red, there being on the average
only one white [pg 028]corpuscle to about
every five hundred of the red ones. On the other hand, the white
corpuscles are larger than the red, one of the former being equal in
volume to about three of the latter.

Fig. 10—Escape of white
corpuscles from a small blood vessel (Hall). At A the conditions are normal, but at B some excitation in the surrounding
tissue leads to a migration of corpuscles. 1, 2, and 3 show different
stages of the passage.

The white corpuscles are found, when studied under favorable
conditions, to possess the power of changing their shape and, by this
means, of moving from place to place. This property enables them to
penetrate the walls of capillaries and to pass with the lymph in between
the cells of the tissues. The white corpuscles are, therefore, not
confined to the blood vessels, as are the red corpuscles, but migrate
through the intercellular spaces (Fig. 10). If any part of the body
becomes inflamed, the white corpuscles collect there in large numbers;
and, on breaking down, they form most of the white portion of the sore,
called the pus.

[pg 029]New white corpuscles are formed
from old ones, by cell-division. Their production may occur in almost
any part of the body, but usually takes place in the lymphatic glands
(Chapter VI) and in the spleen, where conditions for their development
are especially favorable. In these places they are found in great
abundance and in various stages of development.

Functions of White Corpuscles.—The
main use of the white corpuscles appears to be that of a destroyer of
disease germs. These consist of minute organisms that find their way
into the body and, by living upon the tissues and fluids and by
depositing toxins (poisons) in them, cause different forms of disease.
Besides destroying germs that may be present in the blood, the white
corpuscles also leave the blood and attack germs that have invaded the
cells. By forming a kind of wall around any foreign substance, such as a
splinter, that has penetrated the skin, they are able to prevent the
spread of germs through the body. In a similar manner they also prevent
the germs from boils, abscesses, and sore places in general from getting
to and infecting other parts of the body.10
Another function ascribed
to the white corpuscles is that of aiding in the coagulation of the
blood (page 31); and still another, of aiding in the healing of
wounds.

Plasma.—The plasma is a complex
liquid, being made up of water and of substances dissolved in the water.
The dissolved substances consist mainly of foods for the cells and
wastes from the cells.

1. The foods represent the same
classes of materials as are taken in the daily fare, i.e., proteids, carbohydrates,[pg 030]
fats, and salts (Chapter IX).
Three kinds of proteids are found in the plasma, called serum albumin, serum globulin, and fibrinogen. These resemble, in a general way, the white of
raw egg, but differ from each other in the readiness with which they
coagulate. Fibrinogen coagulates more readily than the others and is the
only one that changes in the ordinary coagulation of the blood. The
others remain dissolved during this process, but are coagulated by
chemical agents and by heat. While all of the proteids probably serve as
food for the cells, the fibrinogen, in addition, is a necessary factor
in the coagulation of the blood (page 31).

The only representative of the carbohydrates in the plasma is dextrose. This is a variety of sugar,
being derived from starch and the different sugars that are eaten. The
fat in the plasma is in minute
quantities and appears as fine droplets—the form in which it is found in
milk. While several mineral salts are present in small quantities in the
plasma, sodium chloride, or common
salt, is the only one found in any considerable amount. The mineral
salts serve various purposes, one of which is to cause the proteids to
dissolve in the plasma.

2. The wastes are formed at the
cells, whence they are passed by the lymph into the blood plasma. They
are carried by the blood until removed by the organs of excretion. The
two waste products found in greatest abundance in the plasma are carbon
dioxide and urea.

The substances dissolved in the plasma form about 10 per cent of the
whole amount. The remaining 90 per cent is water. Practically all the
constituents of the plasma, except the wastes, enter the blood from the
digestive organs.

Purposes of Water in the Blood.—Not
only is water the[pg 031] most abundant constituent of the
blood; it is, in some respects, the most important. It is the liquefying
portion of the blood, holding in solution the constituents of the plasma
and floating the corpuscles. Deprived of its water, the blood becomes a
solid substance. Through the movements of the blood the water also
serves the purpose of a transporting agent in the body. The cells in all
parts of the body require water and this is supplied to them from the
blood. Water is present in the corpuscles as well as in the plasma and
forms about 80 per cent of the entire volume of the blood.

Coagulation of the Blood.—If the
blood is exposed to some unnatural condition, such as occurs when it
escapes from the blood vessels, it undergoes a peculiar change known as
coagulation.11
In this change the
corpuscles are collected into a solid mass, known as the clot, thereby separating from a liquid
called the serum. The serum, which is
similar in appearance to the blood plasma, differs from that liquid in
one important respect as explained below.

Causes of Coagulation.—Although
coagulation affects all parts of the blood, only one of its constituents
is found in reality to coagulate. This is the fibrinogen. The formation
of the clot and the separation of the serum is due almost entirely to
the action of this substance. Fibrinogen is for this reason called the
coagulable constituent of the blood.
In the plasma the fibrinogen is in a liquid form; but during coagulation
it changes into a white, stringy solid, called fibrin. This appears in the clot and is the cause of its
formation. Forming as a network of [pg 032]exceedingly fine and very delicate
threads (Fig. 11) throughout the mass of
blood that is coagulating, the fibrin first entangles the
corpuscles and then, by contracting, draws them into the solid mass or
clot.12 The contracting of the fibrin also squeezes out the serum. This
liquid contains all the constituents of the plasma except the
fibrinogen.

Fig. 11—Fibrin
threads (after Ranvier). These by contracting draw the corpuscles
together and form the clot.

Fibrin Ferment and Calcium.—Most
difficult of all to answer have been the questions: What causes the
blood to coagulate outside of the blood vessels and what prevents its
coagulation inside of these vessels? The best explanation offered as yet
upon this point is as follows: Fibrinogen does not of itself change into
fibrin, but is made to undergo this change by the presence of another
substance, called fibrin ferment.
This substance is not a regular constituent of the blood, but is formed
as occasion requires. It is supposed to result from the breaking down of
the white corpuscles, and perhaps also from the blood platelets, when
the blood is exposed to unnatural conditions. The formation of the
ferment leads in turn to the changing of the fibrinogen into fibrin.

Another substance which is necessary to the process of coagulation is
the element calcium. If compounds of calcium are absent from the blood,
coagulation does not take place. These are, however, regular
constituents of healthy blood. Whether the presence of the calcium is
necessary to the formation of the ferment or to the action of the
ferment upon the fibrinogen is unknown.

Purpose of Coagulation.—The purpose
of coagulation is to check the flow of blood from wounds. The fact that
the blood is contained in and kept flowing continuously[pg 033] through a system of connected vessels causes it to escape
rapidly from the body whenever openings in these vessels are made. Clots
form at such openings and close them up, stopping in this way the flow
that would otherwise go on indefinitely. Coagulation, however, does not
stop the flow of blood from the large vessels. From these the blood runs
with too great force for the clot to form within the wound.

Time Required for Coagulation.—The
rate at which coagulation takes place varies greatly under different
conditions. It is influenced strongly by temperature; heat hastens and
cold retards the process. It may be prevented entirely by lowering the
temperature of the blood to near the freezing point. The presence of a
foreign substance increases the rapidity of coagulation, and it has been
observed that bleeding from small wounds is more quickly checked by
covering them with linen or cotton fibers. The fibers in this case
hasten the process of coagulation.

Quantity of Blood.—The quantity of
blood is estimated to be about one thirteenth of the entire weight of
the body. This for the average individual is an amount weighing nearly
twelve pounds and having a volume of nearly one and one half gallons.
About 46 per cent by volume of this amount is made up of corpuscles and
54 per cent of plasma. Of the plasma about 10 per cent consists of
solids and 90 per cent of water, as already stated.

Functions of the Blood.—The blood
is the great carrying, or distributing, agent in the body. Through its
movements (considered in the next chapter) it carries food and oxygen to
the cells and waste materials from the cells. Much of the blood may,
therefore, be regarded as freight in
the process of transportation. The blood also carries, or distributes,
heat. Taking up heat in the warm parts of the body, it gives it off at
places having a lower temperature. This enables all parts of the body to
keep at about the same temperature.

In addition to serving as a carrier, the blood has antiseptic
properties, i.e., it destroys disease germs. While [pg 034] this function is mainly due to the white corpuscles, it is
due in part to the plasma.13 Through its coagulation, the blood also
closes leaks in the small blood vessels. The blood is thus seen to be a
liquid of several functions.

Fig. 12—A balanced
change in water. The level remains constant although the water is
continually changing; suggestive of the changes in the blood.

Changes in the Blood.—In performing
its functions in the body the blood must of necessity undergo rapid and
continuous change. The red corpuscles, whose changes have already been
noted, appear to be the most enduring constituents of the blood. The
plasma is the portion that changes most rapidly. Yet in spite of these
changes the quantity and character of the blood remain practically
constant.14 This is because there is a balancing of the forces that bring about the changes. The
addition of various materials to the blood just equals the withdrawal of
the same materials from the blood. Somewhat as a vessel of water (Fig.
12) having an inflow and an outflow which are equal in amount may keep
always at the same level, the balancing of the intake and outgo of the
blood keeps its composition about the same from time to time.

Hygiene of the Blood.—The blood,
being a changeable liquid, is easily affected through our habits of
living. Since it may be affected for ill as well as for good, one[pg 035] should cultivate those habits that are beneficial and
avoid those that are harmful in their effects. Most of the hygiene of
the blood, however, is properly included in the hygiene of the organs
that act upon the blood—a fact which makes it unnecessary to treat this
subject fully at this time.

From a health standpoint, the most important constituents of the
blood are, perhaps, the corpuscles. These are usually sufficient in
number and vigor in the blood of those who take plenty of physical
exercise, accustom themselves to outdoor air and sunlight, sleep
sufficiently, and avoid the use of injurious drugs. On the other hand,
they are deficient in quantity and inferior in quality in the bodies of
those who pursue an opposite course. Impurities not infrequently find
their way into the blood through the digestive organs. One should eat
wholesome, well-cooked food, drink freely of pure water, and limit the quantity of food to what can be properly digested. The
natural purifiers of the blood are the organs of excretion. The skin is
one of these and its power to throw off impurities depends upon its
being clean and active.

Effect of Drugs.—Certain drugs and
medicines, including alcohol and quinine,15 have recently been shown to
destroy the white corpuscles. The effect of such substances, if
introduced in considerable amount in the body, is to render one less
able to withstand attacks of disease. Many patent medicines are widely
advertised for purifying the blood. While these may possibly do good in
particular cases, the habit of doctoring one’s self with them is open to
serious objection. Instead of taking drugs and patent medicines for
purifying the blood, one should study to live more hygienically. We may
safely rely upon[pg 036] wholesome food, pure water,
outdoor exercise and sunlight, plenty of sleep, and a clean skin for
keeping the blood in good condition. If these natural remedies fail, a
physician should be consulted.

Summary.—The blood is the carrying
or transporting agent of the body. It consists in part of constituents,
such as the red corpuscles, that enable it to carry different
substances; and in part of the materials that are being carried. The
latter, which include food and oxygen for the cells and wastes from the
cells, may be classed as freight. Certain constituents in the blood
destroy disease germs, and other constituents, by coagulating, close
small leaks in the blood vessels. Although subject to rapid and
continuous change, the blood is able—by reason of the balancing of
materials added to and withdrawn from it—to remain about the same in
quantity and composition.

Exercises.—1. Compare blood and water with reference to weight,
density, color, odor, and complexity of composition.

2. Show by an outline the different constituents of the blood.

3. Compare the red and white corpuscles with reference to size,
shape, number, origin, and function.

4. Name some use or purpose for each constituent of the blood.

5. What constituents of the blood may be regarded as freight and what
as agents for carrying this freight?

6. After coagulation, what portions of the blood are found in the
clot? What portions are found in the serum?

7. What purposes are served by water in the blood?

8. Show how the blood, though constantly changing, is kept about the
same in quantity, density, and composition.

9. In the lungs the blood changes from a dark to a bright red color
and in the tissues it changes back to dark red. What is the cause of
these changes?

10. If the oxygen and hemoglobin formed a strong instead of a weak
chemical union, could the hemoglobin then act as an oxygen carrier? Why?

[pg 037]11. What habits of living favor the
development of corpuscles in the blood?

12. Why will keeping the skin clean and active improve the quality of
one’s blood?

CHAPTER V – THE CIRCULATION

A Carrier must move. To enable the blood to carry food and oxygen to the cells and waste materials from the cells, and also to distribute
heat, it is necessary to keep it moving, or circulating, in all parts of
the body. So closely related to the welfare of the body is the
circulation17 of the blood, that its stoppage for only a brief interval
of time results in death.

Discovery of the Circulation.—The
discovery of the circulation of the blood was made about 1616 by an
English physician named Harvey. In 1619 he announced it in his public
lectures and in 1628 he published a treatise in Latin on the
circulation. The chief arguments advanced in support of his views were
the presence of valves in the heart and veins, the continuous movement
of the blood in the same direction through the blood vessels, and the
fact that the blood comes from a cut artery in jets, or spurts, that
correspond to the contractions of the heart.

No other single discovery with reference to the human body has proved
of such great importance. A knowledge of the nature and purpose of the
circulation was the necessary first step in understanding the plan of
the body and the method of maintaining life, and physiology as a science
dates from the time of Harvey’s discovery.

Organs of Circulation.—The organs
of circulation, or blood vessels, are of four kinds, named the heart,
the arteries, the capillaries, and the veins. They serve as [pg 041]contrivances
both for holding the
blood and for keeping it in motion through the body. The heart, which is
the chief organ for propelling the blood, acts as a force pump, while
the arteries and veins serve as tubes for conveying the blood from place
to place. Moreover, the blood vessels are so connected that the blood
moves through them in a regular order, performing two well-defined
circuits.

Fig. 13—Heart in
position in thoracic cavity. Dotted lines show positin of diaphragm and
of margins of lungs.

The Heart.—The human heart, roughly
speaking, is about the size of the clenched fist of the individual
owner. It is situated very near the center of the thoracic cavity and is
almost completely surrounded by the lungs. It is cone-shaped and is so
suspended that the small end hangs downward, forward, and a little to
the left. When from excitement, or other cause, one becomes conscious of
the movements of the heart, these appear to be in the left portion of
the chest, a fact which accounts for the erroneous impression that the
heart is on the left side. The position of the heart in the cavity of
the chest is shown in Fig. 13.

The Pericardium.—Surrounding the
heart is a protective covering, called the pericardium. This consists of
a closed membranous sac so arranged as to form a double covering around
the heart. The heart does not lie inside[pg 042] of the pericardial sac, as seems
at first glance to be the case, but its relation to this space is like
that of the hand to the inside of an empty sack which is laid around it
(Fig. 14). The inner layer of the pericardium is closely attached to the
heart muscle, forming for it an outside covering. The outer layer hangs
loosely around the heart and is continuous with the inner layer at the
top. The outer layer also connects at certain places with the membranes
surrounding the lungs and is attached below to the diaphragm. Between
the two layers of the pericardium is secreted a liquid which prevents
friction from the movements of the heart.

Fig. 14—Diagram of
section of the pericardial sac, heart removed. A. Place occupied by the heart. B. Space inside of pericardial sac. a. Inner layer of pericardium and outer
lining of heart. b. Outer layer of
pericardium. C. Covering of lung. D. Diaphragm.

Cavities of the Heart.—The heart is
a hollow, muscular organ which has its interior divided by partitions
into four distinct cavities. The main partition extends from top to
bottom and divides the heart into two similar portions, named from their
positions the right side and the left side. On each side are two
cavities, the one being directly above the other. The upper cavities are
called auricles and the lower ones
ventricles. To distinguish these
cavities further, they are named from their positions the right auricle
and the left auricle, and the right ventricle and the left ventricle
(Fig. 15). The auricles on each side communicate with the ventricles
below; but after birth there is no communication between the cavities on
the opposite sides of the heart. All the cavities of the heart are lined
with a smooth, delicate membrane, called the endocardium.

Fig. 15—Diagram showing
plan of the heart. 1. Semilunar valves. 2. Tricuspid valve. 3.
Mitral valve. 4. Right auricle. 5. Left auricle. 6. Right ventricle. 7.
Left ventricle. 8. Chordæ tendineæ. 9. Inferior vena cava. 10. Superior
vena cava. 11. Pulmonary artery. 12. Aorta. 13. Pulmonary veins.

[pg 043]Valves of the Heart.—Located at
suitable places in the heart are four gate-like contrivances, called
valves. The purpose of these is to give
the blood a definite direction in its movements. They consist of
tough, inelastic sheets of connective tissue, and are so placed that
pressure on one side causes them to come together and shut up the
passageway, while pressure on the opposite side causes them to open. A
valve is found at the opening of each auricle into the ventricle, and at
the opening of each ventricle into the artery with which it is
connected.

The valve between the right auricle and the right ventricle is called
the tricuspid valve. It is suspended
from a thin ring of connective tissue which surrounds the opening, and
its free margins extend into the ventricle (Fig. 16). It consists of
three parts, as its name implies, which are thrown together in closing
the opening. Joined to the free edges of this valve are many small,
tendinous cords which connect at their lower ends with muscular pillars
in the walls of the ventricle. These are known as the chordæ tendineæ, or heart tendons. Their
purpose is to serve as valve stops,
to prevent the valve from being thrown, by the force of the blood
stream, back into the auricle.

The mitral, or bicuspid, valve is
suspended around the opening between the left auricle and the left
ventricle,[pg 044] with the free margins
extending into the ventricle. It is exactly similar in structure and
arrangement to the tricuspid valve, except that it is stronger and is
composed of two parts instead of three.

Fig. 16—Right side of
heart dissected to show cavities and valves. B. Right semilunar valve. The tricuspid valve and the
chordæ tendineæ shown in the ventricle.

The right semilunar valve is
situated around the opening of the right ventricle into the pulmonary
artery. It consists of three pocket-shaped strips of connective tissue
which hang loosely from the walls when there is no pressure from above;
but upon receiving pressure, the pockets fill and project into the
opening, closing it completely (Fig. 16). The left semilunar valve is around the opening of the left
ventricle into the aorta, and is similar in all respects to the right
semilunar valve.

Differences in the Parts of the
Heart.—Marked differences are found in the walls surrounding the
different cavities of the heart. The walls of the ventricles are much
thicker and stronger than those of the auricles, while the walls of the
left ventricle are two or three times thicker than those of the right. A
less marked but similar difference exists between the auricles and also
between the valves on the two sides of the heart. These differences in
structure are all accounted for by the work done by the different
portions of the heart. The greater the work, the heavier the structures
that perform the work.

Fig. 17—Diagram of the circulation, showing in
general the work done by each part of the heart. The right ventricle
forces the blood through the lungs and into the left auricle. The left
ventricle forces blood through all parts of the body and back to the
auricle. The auricles force blood into the ventricles.

[pg 045]Connection with Arteries and
Veins.—Though the heart is in communication with all parts of the
circulatory system, it makes actual connection with only a few of the
blood tubes. These enter the heart at its upper portion (Fig. 15), but
connect with its different cavities as follows:

1. With the right auricle, the
superior and the inferior venæ cavæ and the coronary veins. The superior
vena cava receives blood from the head and the upper extremities; the
inferior vena cava, from the trunk and the lower extremities; and the
coronary veins, from the heart itself.

2. With the left auricle, the four
pulmonary veins. These receive blood from the lungs and empty it into
the left auricle.

3. With the right ventricle, the
pulmonary artery. This receives blood from the heart and by its branches
distributes it to all parts of the lungs.

4. With the left ventricle, the
aorta. The aorta receives blood from the heart and through its branches
delivers it to all parts of the body.

How the Heart does its Work.—The
heart is a muscular pump18 and does its work through the contracting
and[pg 046] relaxing of its walls. During
contraction the cavities are closed and the blood is forced out of them.
During relaxation the cavities open and are refilled. The valves direct
the flow of the blood, being so arranged as to keep it moving always in
the same direction (Fig. 17).

The heart, however, is not a single or a simple pump, but consists in
reality of four pumps which
correspond to its different cavities. These connect with each other and
with the blood vessels over the body in such a manner that each aids in
the general movement of the blood.

Fig. 18—Diagram illustrating the “cardiac
cycle.”

Work of Auricles and Ventricles
Compared.—In the work of the heart the two auricles contract at the
same time—their contraction being followed immediately by the
contraction of both ventricles. After the contraction of the ventricles
comes a period of rest, or relaxation, about equal in time to the period
of contraction of both the auricles and the ventricles.19 On account of
the work which they perform, the auricles have been called the “feed
pumps” of the heart; and the ventricles, the “force pumps.”20 It is the
function of the auricles to collect the blood from the veins, to let
this run slowly into the ventricles when both the[pg 047] auricles and ventricles are
relaxed, and finally, by contracting, to
force an excess of blood into the ventricles, thereby distending
their walls. The ventricles, having in this way been fully charged by
the auricles, now contract and force their contents into the large
arteries.

Sounds of the Heart.—Two distinct
sounds are given out by the heart as it pumps the blood. One of them is
a dull and rather heavy sound, while the other is a short, sharp sound.
The short sound follows quickly after the dull sound and the two are
fairly imitated by the words “lūbb, dŭp.” While the cause of the
first sound is not fully understood, most authorities believe it to be
due to the contraction of the heart muscle and the sudden tension on the
valve flaps. The second sound is due to the closing of the semilunar
valves. These sounds are easily heard by placing an ear against the
chest wall. They are of great value to the physician in determining the
condition of the heart.

Arteries and Veins.—These form two
systems of tubes which reach from the heart to all parts of the body.
The arteries receive blood from the heart and distribute it to the
capillaries. The veins receive the blood from the capillaries and return
it to the heart. The arteries and veins are similar in structure, both
having the form of tubes and both having three distinct layers, or
coats, in their walls. The corresponding coats in the arteries and veins
are made up of similar materials, as follows:

1. The inner coat consists of a
delicate lining of flat cells resting upon a thin layer of connective
tissue. The inner coat is continuous with the lining of the heart and
provides a smooth surface over which the blood glides with little
friction.

2. The middle coat consists mainly
of non-striated, or involuntary, muscular fibers. This coat is quite
thin in the veins, but in the arteries it is rather thick and
strong.

3. The outer coat is made up of a
variety of connective[pg 048] tissue and is also much thicker
and stronger in the arteries than in the veins.

Fig. 19—Artery dissected to show the coats.

Marked differences exist between the arteries and the veins, and
these vessels are readily distinguished from each other. The walls of
the arteries are much thicker and heavier than those of the veins (Fig.
19). As a result these tubes stand open when empty, whereas the veins
collapse. The arteries also are highly elastic, while the veins are but
slightly elastic. On the other hand, many of the veins contain valves,
formed by folds in the inner coat (Fig. 20), while the arteries have no
valves. The blood flows more rapidly through the arteries than through
the veins, the difference being due to the fact that the system of veins
has a greater capacity than the system of arteries.

Fig. 20—Vein split open to show the valves.

Why the Arteries are Elastic.—The
elasticity of the arteries serves a twofold purpose. It keeps the
arteries from bursting when the blood is forced into them from the
ventricles, and it is a means of supplying
pressure to the blood while the ventricles are in a condition of
relaxation. The latter purpose is accomplished as follows:

Contraction of the ventricles fills the arteries overfull, causing
them to swell out and make room for the excess of blood. Then while the
ventricles are resting and filling, the stretched arteries press upon
the blood to keep it[pg 049] flowing into
the capillaries. In this way they cause
the intermittent flow from, the heart to become a steady stream in the
capillaries.

The swelling of the arteries at each contraction of the ventricle is
easily felt at certain places in the body, such as the wrist. This
expansion, known as the “pulse,” is the chief means employed by the
physician in determining the force and rapidity of the heart’s
action.

Purpose of the Valves in the
Veins.—The valves in the veins are not used for directing the general flow of the blood, the valves of
the heart being sufficient for this purpose. Their presence is necessary
because of the pressure to which the veins are subjected in different
parts of the body. The contraction of a muscle will, for example, close
the small veins in its vicinity and diminish the capacity of the larger
ones. The natural tendency of such pressure is to empty the veins in two
directions—one in the same direction as the regular movement of the
blood, but the other in the opposite direction. The valves by closing
cause the contracting muscle to push the blood in one direction
only—toward the heart. The valves in the veins are, therefore, an
economical device for enabling variable
pressure in different parts of the body to assist in the circulation. Veins like the inferior vena
cava and the veins of the brain, which are not compressed by movements
of the body, do not have valves.

Purposes of the Muscular Coat.—The
muscular coat, which is thicker in the arteries than in the veins and is
more marked in small arteries than in large ones, serves two important
purposes. In the first place it, together with the elastic tissue, keeps
the capacity of the blood vessels equal to
the volume of the blood. Since the blood vessels are capable of
holding more blood than may be[pg 050] present at a given time in the
body, there is a liability of empty spaces occurring in these tubes.
Such spaces would seriously interfere with the circulation, since the
heart pressure could not then reach all parts of the blood stream. This
is prevented by the contracted state, or “tone,” of the blood vessels,
due to the muscular coat.

In the second place, the muscular coat serves the purpose of regulating the amount of blood which any
given organ or part of the body receives. This it does by varying the
caliber of the arteries going to the organ in question. To increase the
blood supply, the muscular coat relaxes. The arteries are then dilated
by the blood pressure from within so as to let through a larger quantity
of blood. To diminish the supply, the muscle contracts, making the
caliber of the arteries less, so that less blood can flow to this part
of the body. Since the need of organs for blood varies with their
activity, the muscular coat serves in this way a very necessary
purpose.

Fig. 21—Diagram of network of capillaries between a
very small artery and a very small vein. Shading indicates the change of
color of the blood as it passes through the capillaries. S. Places between capillaries occupied by
the cells.

Capillaries.—The capillaries
consist of a network of minute blood vessels which connect the
terminations of the smallest arteries with the beginnings of the
smallest veins (Fig. 21). They have an average diameter of less than one
two-thousandth of an inch (12 µ) and an average length of less than one
twenty-fifth of an inch (1 millimeter). Their walls consist of a single
[pg 051] coat which is continuous with the
lining of the arteries and veins. This coat is formed of a single layer
of thin, flat cells placed edge to edge (Fig. 22). With a few
exceptions, the capillaries are found in great abundance in all parts of
the body.

Fig. 22—Surface of
capillary highly magnified, showing its coat of thin cells placed
edge to edge.

Functions of the Capillaries.—On
account of the thinness of their walls, the capillaries are able to
serve a twofold purpose in the body:

1. They admit materials into the blood vessels.

2. They allow materials to pass from the blood vessels to the
surrounding tissues.

When it is remembered that the blood, as blood, does not escape from
the blood vessels under normal conditions, the importance of the work of
the capillaries is apparent. To serve its purpose as a carrier, there
must be places where the blood can load up with the materials which it
is to carry, and places also where these can be unloaded. Such places
are supplied by the capillaries.

The capillaries also serve the purpose of spreading the blood out and
of bringing it very near the individual cells in all parts of the body
(Fig. 21).

Functions of Arteries and
Veins.—While the capillaries provide the means whereby materials
may both enter and leave the blood, the arteries and veins serve the
general purpose of passing the blood from one set of capillaries to
another. Since pressure is necessary for moving the blood, these tubes
must connect with the source of the pressure, which is the heart. In the
arteries and veins the blood neither receives nor gives up material, but
having received or given up material at one set of capillaries, it is
then pushed through these tubes to where it can serve a similar purpose
in another set of capillaries (Fig. 23).

Divisions of the Circulation.—Man,
in common with all warm-blooded animals, has a double circulation, a
fact[pg 052] which explains the double
structure of his heart. The two divisions are known as the pulmonary and the systemic circulations. By the former the blood passes from
the right ventricle through the lungs, and is then returned to the left
auricle; by the latter it passes from the left ventricle through all
parts of the body, returning to the right auricle.

The general plan of the circulation is indicated in Fig. 23. All the
blood flows continuously through both circulations and passes the
various parts in the following order: right auricle, tricuspid valve,
right ventricle, right semilunar valve, pulmonary artery and its
branches, capillaries of the lungs, pulmonary veins, left auricle,
mitral valve, left ventricle, left semilunar valve, aorta and its
branches, systemic capillaries, the smaller veins, superior and inferior
venæ cavæ, and then again into the right auricle.

In the pulmonary capillaries the blood gives up carbon dioxide and
receives oxygen, changing from a dark red to a bright red color. In the
systemic capillaries it gives up oxygen, receives carbon dioxide and
other impurities, and changes back to a dark red color.

In addition to the two main divisions of the circulation, special
circuits are found in various places. Such a circuit in the liver is
called the portal circulation, and
another in the kidneys is termed the renal circulation. To some extent the blood supply to the
walls of the heart is also outside of the general movement; it is called
the coronary circulation.

Fig. 23—General scheme of the
circulation, showing places where the blood takes on and gives off
materials. 1. Body in general. 2. Lungs. 3. Kidneys. 4. Liver. 5. Organs
of digestion. 6. Lymph ducts. 7. Pulmonary artery. 8. Aorta.

Blood Pressure and Velocity.—The
blood, in obedience to physical laws, passes continuously through the
blood vessels, moving always from a place of greater to one of less
pressure. Through the contraction of the ventricles, a relatively high
pressure is maintained in the arteries nearest the heart.21 This pressure diminishes
rapidly in the[pg 054] small arteries,
becomes comparatively slight in the capillaries, and falls practically
to nothing in the veins. Near the heart in the superior and inferior
venæ cavæ, the pressure at intervals is said to be negative. This means that the blood from these veins is
actually drawn into the right auricle by the expansion of the chest
walls in breathing.22

The velocity of the blood is greatest in the arteries, less in the
veins, and much less in the
capillaries than in either the arteries or the veins. The slower flow of
the blood through the capillaries is accounted for by the fact that
their united area is many times greater than that of the arteries which
supply, or the veins which relieve, them. This allows the same quantity
of blood, flowing through them in a given time, a wider channel and
causes it to move more slowly. The time required for a complete
circulation is less than one minute.

Summary of Causes of Circulation.—The
chief factor in the circulation of the blood is, of course, the
heart. The ventricles keep a pressure on the blood which is sufficient
to force it through all the blood tubes and back to the auricles. The
heart is aided in its work by the elasticity of the arteries, which
keeps the blood under pressure while the ventricles are in a state of
relaxation. It is also aided by the muscles and elastic tissue in all of
the blood vessels. These, by keeping the blood vessels in a state of
“tone,” or so contracted that their capacity just equals the volume of
the blood, enable pressure from the heart to be transmitted to all parts
of the blood stream. A further aid to the circulation is found in the
valves in the veins, which enable muscular contraction within the body,
and variable pressure upon its surface, to drive the blood toward the
heart. The heart is also aided to some extent by the movements of the
chest walls in breathing. The organs Of circulation are under the
control of the nervous system (Chapter XVIII).

PRACTICAL WORK

In showing the relations of the different parts of the heart, a large
dissectible model is of great service (Fig. 24). Indeed, where the time
of the class is limited, the practical work may be confined to the study
of the heart model, diagrams of the heart and the circulation, and a few
simple experiments. However, where the course is more extended, the
dissection of the heart of some animal as described below is strongly
advised.

Observations on the Heart.—Procure,
by the assistance of a butcher, the heart of a sheep, calf, or hog. To
insure the specimen against mutilation, the lungs and the diaphragm must
be left attached to the heart. In studying the different parts, good
results will be obtained by observing the following order:

1. Observe the connection of the heart to the lungs, diaphragm, and
large blood vessels. Inflate the lungs and observe the position of the
heart with reference to them.

2. Examine the sac surrounding the heart, called the pericardium. Pierce its lower portion and
collect the pericardial fluid. Increase the [pg 061]opening thus made until it is large enough to slip the
heart out through it. Then slide back the pericardium until its
connection with the large blood vessels above the heart is found.
Observe that a thin layer of it continues down from this attachment,
forming the outer covering of the heart.

3. Trace out for a short distance and study the veins and arteries
connected with the heart. The arteries are to be distinguished by their
thick walls. The heart may now be severed from the lungs by cutting the
large blood vessels, care being taken to leave a considerable length of
each one attached to the heart.

Fig. 24—Model for demonstrating the heart.

4. Observe the outside of the heart. The thick, lower portion
contains the cavities called ventricles; the thin, upper, ear-shaped portions are the
auricles. The thicker and denser side
lies toward the left of the animal’s body and is called the left side of the heart; the other is the
right side. Locate the right auricle
and the right ventricle; the left auricle and the left ventricle.

5. Lay the heart on the table with the front side up and the apex
pointing from the operator. This places the left side of the heart to
his left and the right side to his right. Notice the groove between the
ventricles, called the inter-ventricular groove. Make an incision half
an inch to the right of this groove and cut toward the base of the heart
until the pulmonary artery is laid open. Then, following within half an
inch of the groove, cut down and around the right side of the heart. The
wall of the right ventricle may now be raised and the cavity exposed.
Observe the extent of the cavity, its shape, its lining, its columns of
muscles, its half columns of muscles, its tendons (chordæ tendineæ), the
tricuspid valve from the under side, etc. Also notice the valve at the
beginning of the pulmonary artery (the right semilunar) and the sinuses,
or depressions, in the artery immediately behind its divisions.

6. Now cut through the middle of the loosened ventricular wall from
the apex to the middle of the right auricle, laying it open for
[pg 062]observation. Observe the
openings into the auricle, there being one each for the vena cava
superior, the vena cava inferior, and the coronary vein. Compare the
walls, lining, shape, size, etc., with the ventricle below.

7. Cut off the end of the left ventricle about an inch above the
apex. This will show the extension of the cavity to the apex; it will
also show the thickness of the walls and the shape of the cavity. Split
up the ventricular wall far enough to examine the mitral valve and the
chordæ tendineæ from the lower side.

8. Make an incision in the left auricle. Examine its inner surface
and find the places of entrance of the pulmonary veins. Examine the
mitral valve from above. Compare the two sides of the heart, part for
part.

9. Separate the aorta from the other blood vessels and cut it
entirely free from the heart, care being taken to leave enough of the
heart attached to the artery to insure the semilunar valve’s being left
in good condition. After tying or plugging up the holes in the sides of
the artery, pour water into the small end and observe the closing of the
semilunar valve. Repeat the experiment until the action of the valve is
understood. Sketch the artery, showing the valve in a closed
condition.

To illustrate the Action of a
Ventricle.—Procure a syringe bulb with an opening at each end.
Connect a rubber tube with each opening, letting the tubes reach into
two tumblers containing water. By alternately compressing and releasing
the bulb, water is pumped from one vessel into the other. The bulb may
be taken to represent one of the ventricles. What action of the
ventricle is represented by compressing the bulb? By releasing the
pressure? Show by a sectional drawing the arrangement of the valves in
the syringe bulb.

Fig. 25—Illustrating elasticity of arteries.

To show the Advantage of the Elasticity
of Arteries.—Connect the syringe bulb used in the last experiment
with a rubber tube three or four feet in length and having rather thin
walls. In the opposite end of the rubber tube insert a short glass tube
which has been drawn (by heating) to a fine point (Fig. 25). Pump water
into the rubber tube, observing:

1. The swelling of the tube (pulse) as the water is forced into it.
(This is best observed by placing the fingers on the tube.)

[pg 063]2. The forcing of water from the
pointed tubs during the interval when no pressure is being applied from
the bulb. Compare with the action of the arteries when blood is forced
into them from the ventricles.

Repeat the experiment, using a long glass tube terminating in a point
instead of the rubber tube. (In fitting the glass tube to the bulb use a
very short rubber tube.) Observe and account for the differences in the
flow of water through the inelastic tube.

To show the Advantage of Valves in the
Veins.—Attach an open glass tube one foot in length to each end of
the rubber tube used in the preceding experiment and fill with water (by
sucking) to within about six inches of the end. Lay on the table with
the glass tubes secured in an upright position (Fig. 26). Now compress
the tube with the hand, noting that the water rises in both tubes, being
pushed in both directions. This effect is similar to that produced on
the blood when a vein having no valves is compressed.

Fig. 26.—Simple
apparatus for showing advantage of valves in veins.

Now imitate the action of a valve by clamping the tube at one point,
or by closing it by pressure from the finger, and then compressing with
the hand some portion of the tube on the table. Observe in this instance
that the water is all pushed in the
same direction. The movement of the water is now like the effect
produced on the blood in veins having valves when the veins are
compressed.

To show the Position of the Valves in
the Veins.—Exercise the arm and hand for a moment to increase the
blood supply. Expose the forearm and examine the veins on its surface.
With a finger, stroke one of the veins toward the heart, noting that, as
the blood is pushed along on one side of the finger the blood follows on
the other side. Now stroke the vein toward the hand. Places are found
beyond which the blood does not follow the finger. These mark the
positions of valves.

To show Effect of Exercise upon the
Circulation.—1. With a finger on the “pulse” at the wrist or
temple, count the number of heart beats during a period of one minute
under the following conditions: (a)
when sitting; (b) when standing; (c) after active exercise, as running.
What relation, if any, do these observations indicate between the
general activity of the body and the work of the heart?

2. Compare the size of the veins on the backs of the hands when they
are placed side by side on a table. Then exercise briskly the [pg 064]right hand and arm, clenching and
unclenching the fist and flexing the arm at the elbow. Place the hands
again side by side and, after waiting a minute, observe the increase in
the size of the veins in the hand exercised. How is this accounted
for?

To Show the Effect of Gravity on the
Circulation.—Hold one hand high above the head, at the same time
letting the other hand hang loosely by the side. Observe the difference
in the color of the hands and the degree to which the large veins are
filled. Repeat the experiment, reversing the position of the hands. What
results are observed? In what parts of the body does gravity aid in the
return of the blood to the heart? In what parts does it hinder? Where
fainting is caused by lack of blood in the brain (the usual cause), is
it better to let the patient lie down flat or to force him into a
sitting posture?

To study the Circulation in a Frog’s
Foot (Optional).—A compound microscope is needed in this study and
for extended examination it is best to destroy the frog’s brain. This is
done by inserting some blunt-pointed instrument into the skull cavity
from the neck and moving it about. A small frog, on account of the
thinness of its webs, gives the best results. It should be attached to a
thin board which has an opening in one end over which the web of the
foot may be stretched. Threads should extend from two of the toes to
pins driven into the board to secure the necessary tension of the web,
and the foot and lower leg should be kept moist. Using a two-thirds-inch
objective, observe the branching of the small arteries into the
capillaries and the union of the capillaries to form the small veins.
The appearance is truly wonderful, but allowance must be made for the
fact that the motion of the blood is
magnified, as well as the different structures, and that it appears to
move much faster than it really does. With a still higher power, the
movements of the corpuscles through the capillaries may be studied.

Note.—To perform this experiment without destroying the brain, the
frog is first carefully wrapped with strips of wet cloth and securely
tied to the board. The wrapping, while preventing movements of the frog,
must not interfere with the circulation.

CHAPTER VI – THE LYMPH AND ITS MOVEMENT THROUGH THE BODY

Fig. 27—Diagram showing
position of the lymph with reference to the blood and the cells.
The central tube is a capillary. The arrows indicate the direction of
slight movements in the lymph.

The blood, it will be remembered, moves everywhere through the body
in a system of closed tubes. These
keep it from coming in contact with any of the cells of the body except
those lining the tubes themselves. The capillaries, to be sure, bring
the blood very near the cells of the different tissues; still, there is
need of a liquid to fill the space between the capillaries and the cells
and to transfer materials from one to the other. The lymph occupies this
position and does this work. The position of the lymph with reference to
the capillaries and the cells is shown in Fig. 27.

Origin of the Lymph.—The chief
source of the lymph is the plasma of the blood. As before described, the
walls of the capillaries consist of a single layer of flat cells placed
edge to edge. Partly on account of the pressure upon the blood and
partly on account of the natural tendency of liquids to pass through
animal membranes, a considerable portion of the plasma penetrates the
thin walls and enters the spaces occupied by the lymph.

[pg 066]The cells themselves also help to form the lymph, since the water and
wastes leaving the cells add to its bulk. These mix with the plasma from
the blood, forming the resultant liquid which is the lymph. A
considerable amount of the material absorbed from the food canal also
enters the lymph tubes, but this passes into the blood before reaching
the cells.

Composition and Physical Properties of
the Lymph.26—As would naturally be expected, the composition of
the lymph is similar to that of the blood. In fact, nearly all the
important constituents of the blood are found in the lymph, but in
different proportions. Food materials for the cells are present in
smaller amounts than in the blood, while impurities from the cells are
in larger amounts. As a rule the red corpuscles are absent from the
lymph, but the white corpuscles are present and in about the same
numbers as in the blood.

The physical properties of the lymph are also similar to those of the
blood. Like the blood, the lymph is denser than water and also
coagulates, but it coagulates more slowly than does the blood. The most
noticeable difference between these liquids is that of color, the lymph
being colorless. This is due to the absence of red corpuscles. The
quantity of lymph is estimated to be considerably greater than that of
the blood.

Lymph Vessels.—Most of the lymph
lies in minute cavities surrounding the cells and in close relations
with the capillaries (Figs. 27 and 30). These are called lymph spaces. Connecting with the lymph
spaces on the one[pg 067] hand, and with certain blood
vessels on the other, is a system of tubes that return the lymph to the
blood stream. The smallest of these, and the ones in greatest abundance,
are called lymphatics. They consist
of slender, thin-walled tubes, which resemble veins in structure, and,
like the veins, have valves. They differ from veins, however, in being
more uniform in size and in having thinner walls.

Fig. 28—Diagram of
drainage system for the lymph. 1. Thoracic duct. 2. Right
lymphatic duct. 3. Left subclavian vein. 4. Right subclavian vein. 5.
Superior vena cava. 6. Lacteals. 7. Lymphatic glands. The small tubes
connecting with the lymph spaces in all parts of the body are the
lymphatics.

The lymphatics in different places gradually converge toward, and
empty into, the two main lymph tubes of the body. The smaller of these
tubes, called the right lymphatic
duct, receives the lymph from the lymphatics in the right arm, the
right side of the head, and the region of the right shoulder. It
connects with, and empties its contents into, the right subclavian vein
at the place where it is joined by the right jugular vein (Fig. 28).

The larger of the lymph tubes is called the thoracic duct. This receives lymph from all parts of the
body[pg 068] not drained by the right
lymphatic duct, and empties it into the left subclavian vein. Connection
is made with the subclavian vein on the upper side at the place where it
is joined by the left jugular vein. The thoracic duct has a length of
from sixteen to eighteen inches, and is about as large around as a goose
quill. The lower end terminates in an enlargement in the abdominal
cavity, called the receptacle of the
chyle. It is provided with valves throughout its course, in
addition to one of considerable size which guards the opening into the
blood vessel.

The lymphatics which join the thoracic duct from the small intestine
are called the lacteals (Fig. 28).
These do not differ in structure from the lymphatics in other parts of
the body, but they perform a special work in absorbing the digested fat
(Chapter XI).

Lymphatic Glands.—The lymphatic
glands, sometimes called lymph nodes, are small and somewhat rounded
bodies situated along the course of the lymphatic tubes. They vary in
size, some of them being an inch or more in length. The lymph vessels
generally open into them on one side and leave them on the other (Figs.
28 and 30). They are not glands in function, but are so called because
of their having the general form of glands. They provide favorable
conditions for the development of white corpuscles (page 29). They also
separate harmful germs and poisonous wastes from the lymph, thereby
preventing their entrance into the blood.

Relations of the Lymph, the Blood, and
the Cells.—While the blood is necessary as a carrying, or
transporting, agent in the body, the lymph is necessary for transferring
materials from the blood to the cells and vice versa. Serving as a physiological “go between,” or
medium of exchange, the lymph enables the blood to minister to the[pg 069] needs of the cells. But the lymph
and the blood, everything considered, can hardly be looked upon as two
separate and distinct liquids. Not only do they supplement each other in
their work and possess striking similarities, but each is made in its
movements to pass into the vessels occupied by the other, so that they
are constantly mixing and mingling. For these and other reasons, they
are more properly regarded as two divisions of a single liquid—one
which, by adapting itself to different purposes,27 supplies all the
conditions of a nutrient fluid for the cells.

Movements of the Lymph.—As compared
with the blood, the lymph must be classed as a quiet liquid. But, as
already suggested, it has certain movements which are necessary to the
purposes which it serves. A careful study shows it to have three
well-defined movements as follows:

1. A movement from the capillaries toward the cells.

2. A movement from the cells toward the capillaries.

3. A movement of the entire body of lymph from the lymph spaces into
the lymphatics and along these channels to the ducts through which it
enters the blood.

By the first movement the cells receive their nourishment. By the
second and third movements the lymph, more or less laden with
impurities, is returned to the blood stream. (See Figs. 28 and 30.)

Causes of the Lymph Movements.—Let
us consider first the movement through the lymph tubes. No pump, like
the heart, is known to be connected with these tubes and[pg 070] to supply the pressure necessary
for moving the lymph. There are, however, several forces that indirectly
aid in its flow. The most important of these are as follows:

1. Blood Pressure at the
Capillaries.—The plasma which is forced through the capillary walls
by pressure from the heart makes room for itself by pushing a portion of
the lymph out of the lymph spaces. This in turn presses upon the lymph
in the tubes which it enters. In this way pressure from the heart is
transmitted to the lymph, forcing it to move.

2. Variable Pressure on the Walls of
the Lymph Vessels.—Pressure exerted on the sides of the lymph tubes
by contracting muscles tends to close them up and to push the lymph past
the valves, which, by closing, prevent its return (Fig. 29). Pressure at
the surface of the body, provided that it is variable, also forces the
lymph along. The valves in the lymph vessels serve the same purpose as
those in the veins.

Fig. 29—Diagram to
show how the muscles pump lymph. A.
Relaxed muscle beside which is a lymphatic tube. B. Same muscle in state of contraction.

3. The Inspiratory Force.—When the
thoracic cavity is enlarged in breathing, the unbalanced atmospheric
pressure is exerted from all directions towards the thoracic space. This
not only causes the air to flow into the lungs (Chapter VII), but also
causes a movement of the blood and lymph in such of their tubes as enter
this cavity. It will be noted that both of the large lymph ducts
terminate where their contents may be influenced by the respiratory
movements. (See Practical Work.)

Where the Lymph enters the
Blood.—The fact that the lymph is poured into the blood at but two
places, and these very close to each[pg 071]
other, requires a word of explanation. As a matter of fact, it is
impossible for the lymph to flow into blood vessels at most places on
account of the blood pressure. This would force the blood into the lymph
vessels, instead of allowing the lymph to enter the blood. The lymph can
enter only at some place where the blood pressure is less than the
pressure that moves the lymph. Such a place is found in the thoracic
cavity. As already pointed out (page 54), the blood pressure in the
veins entering this cavity becomes, with each expansion of the chest,
negative, i.e., less than the pressure of the atmosphere on the outside
of the body. This, as we have seen, aids in the flow of the blood into
the right auricle. It also aids in the passage of lymph into the blood
vessels. The lymph is said to be “sucked in,” which means that it is
forced in by the unbalanced pressure of the atmosphere.28 Some
advantage is also gained by the lymph duct’s entering the subclavian
vein on the upper side and at its union with the jugular vein.
Everything considered, it is found that the lymph flows into the blood
vessels where it can be “drawn in” by the movements of breathing and
where it meets with no opposition from the blood stream itself (Fig.
30).

Fig. 30—Diagram
showing general movement of lymph from the place of relatively high
pressure at the lymph spaces to the place of relatively low pressure in
the thoracic cavity.

Lymph Movements at the Cells.—The
double movement of the lymph from the capillaries toward the cells[pg 072] and from the cells
toward the capillaries is not entirely understood. Blood pressure in the
capillaries undoubtedly has much to do in forcing the plasma through the
capillary walls, but this tends to prevent the movement of the lymph in
the opposite direction. Movements between the blood and the lymph are
known to take place in part according to a general principle, known as
osmosis, or dialysis.

Fig. 31—Vessel with
an upright membranous partition for illustrating osmosis.

Osmosis.—The term “osmosis” is used
to designate the passage of liquids through some partition which
separates them. Thus, if a vessel with an upright membranous partition
be filled on the one side with pure water and on the other with water
containing salt, an exchange of materials will take place through the
membrane until the same proportion of salt exists on the two sides (Fig.
31). The cause of osmosis is the motion of the molecules, or minute
particles, that make up the liquid substance. If the partition were not
present, this motion would simply cause a mixing of the liquids.

Conditions under which Osmosis
occurs.—Osmosis may be shown by suitable experiments (see Practical
Work) to take place under the following conditions:

1. The liquids on the two sides of the partition must be unlike either in density or in
composition. Since the effect of the movement is to reduce the liquids
to the same condition, a difference in
density causes the flow to be greater from the less dense toward the
denser liquid, than in the opposite direction; while a difference in composition causes the
substances in solution to move from the place of greater abundance
toward places of less abundance.

2. The liquids must be capable of wetting, or penetrating, the
partition. If but one of the liquids penetrates the partition, the flow
will be in but one direction.

3. The liquids on the two sides of the partition must readily mix
with each other.

Osmosis at the Cells.—In the body
osmosis takes place between the[pg 073]
blood and the lymph and between the lymph and the cells, the movements
being through the capillary walls and the membranes inclosing the cells
(Fig. 27). Oxygen and food materials, which are found in great abundance
in the blood, are less abundant in the lymph and still less abundant in
the cells. According to the principle of osmosis, the main flow of
oxygen and food is from the capillaries toward the cells. On the other
hand, the wastes are most abundant in the cells where they are formed,
less abundant in the lymph, and least abundant in the blood. Hence the
wastes flow from the cells toward the capillaries.

Solutions.—Neither the blood plasma
nor the lymph, as already shown, are simple liquids; but they consist of
water and different substances dissolved in the water. They belong to a
class of substances called solutions.
The chief point of interest about substances in solution is that they
are very finely divided and that their little particles are free to move
about in the liquid that contains them. Both the motion and the finely
divided condition of the dissolved substances are necessary to the
process of osmosis. All substances, however, that appear to be in
solution are not able to penetrate membranes, or take part in
osmosis.

Kinds of Solutions in the Body.—The
substances in solution in the body liquids are of two general kinds
known as colloids and crystalloids. The crystalloids are able
to pass through membranous partitions, while the colloids are not. An
example of a colloid is found in the albumin of an egg, which is unable
to penetrate the membrane which surrounds it. Examples of crystalloids
are found in solutions of salt and sugar in water. The inability of a
colloid to penetrate a membrane is due to the fact that it does not form
a true solution. Its particles (molecules), instead of being completely
separated, still cling together, forming little masses that are too
large to penetrate the membrane. Since, however, it has the appearance,
on being mixed with water, of being dissolved, it is called a colloidal solution. The crystalloid
substance, on the other hand, completely separates in the water and
forms a true solution—one which is
able to penetrate the partition or membrane.

Osmosis not a Sufficient Cause.—The
passage of materials through animal membranes, according to the
principle of osmosis, is limited to crystalloid substances. But colloid
substances are also known to pass through the various partitions of the
body. An example of such is found in the proteids of the blood which, as
a colloidal solution, pass through the capillary walls to become a part
of the lymph. Perhaps[pg 074] the best explanation offered as
yet for this passage is that the colloidal substances are changed by the
cells lining the capillaries into substances that form true solutions
and that after the passage they are changed back again to the colloidal
condition.

Summary.—Between the cells and the
capillaries is a liquid, known as the lymph, which is similar in
composition and physical properties to the blood. It consists chiefly of
escaped plasma. The vessels that contain it are connected with the
system for the circulation of the blood. By adding new material to the
lymph and withdrawing waste material from it, the blood keeps this
liquid in a suitable condition for supplying the needs of the cells.
Supplementing each other in all respects, the blood and the lymph
together form the nutrient cell fluid of the body. The interchange of
material between the blood and the lymph, and the lymph and the cells,
takes place in part according to the principle of osmosis.

Exercises.—1. Explain the necessity
for the lymph in the body.

2. Compare lymph and water with reference to density, color, and
complexity of composition.

3. Compare lymph and blood with reference to color, composition, and
movement through the body.

4. Show how blood pressure in the capillaries causes a flow of the
lymph.

5. Show how contracting muscles cause the lymph to move. Compare with
the effect of muscular contraction upon the blood in the veins.

6. Trace the lymph in its flow from the right hand to where it enters
the blood; from the feet to where it enters the blood.

7. What conditions prevail at the cells to cause a movement of food
and oxygen in one direction and of waste materials in the opposite
direction?

8. What part does water play in the exchanges at the cells?

9. Show that the blood and the lymph together fulfill all the
requirements of a nutrient cell fluid in the body.

[pg 075]

PRACTICAL WORK

To illustrate the Effect of Breathing
upon the Flow of Lymph.—Tightly holding one end of a glass tube
between the lips, let the other end extend into water in a tumbler on a
table. In this position quickly inhale air through the nostrils, noting
that with each inhalation there is a slight movement of the water up the
tube. (No sucking action should be exerted by the mouth.) Apply to the
movements in the large blood and lymph vessels entering the thoracic
cavity.

To illustrate Osmosis.—1. Separate
the shell from the lining membrane at one end of an egg, over an area
about one inch in diameter. To do this without injuring the membrane,
the shell must first be broken into small pieces and then picked off
with a pair of forceps, or a small knife blade. Fit a small glass tube,
eight or ten inches long, into the other end so that it will penetrate
the membrane and pass down into the yolk. Securely fasten the tube to
the shell by melting beeswax around it, and set the egg in a small
tumbler partly filled with water. Examine in the course of half an hour.
What evidence now exists that the water has passed through the
membrane?

2. Tie over the large end of a “thistle tube” (used by chemists) a
thin animal membrane, such as a piece of the pericardium or a strip of
the membrane from around a sausage. Then fill the bulb and the lower end
of the tube with a concentrated solution of some solid, such as sugar,
salt, or copper sulphate. Suspend in a vessel of water so that the
liquid which it contains is just on a level with the water in the
vessel. Examine from time to time, looking for evidence of a movement in
each direction through the membrane. Why should the movement of the
water into the tube be greater than the movement in the opposite
direction? (If the thistle tube has a very slender stem, it is better to
fill the bulb before tying on the membrane. The opening in the stem may
be plugged during the process of filling.)

Fig. 32—An osmosometer.

Note.—With a special piece of apparatus, known as an osmosometer, the principle of osmosis may
be more easily illustrated than by the method in either of the above
experiments (Fig. 32). This apparatus may be obtained from supply
houses.

CHAPTER VII – RESPIRATION

Through the movements of the blood and the lymph, materials entering
the body are transported to the cells, and wastes formed at the cells
are carried to the organs which remove them from the body. We are now to
consider the passage of materials from outside the body to the cells and
vice versa. One substance which the
body constantly needs is oxygen, and one which it is constantly throwing
off is carbon dioxide. Both of these are constituents of

The Atmosphere.—The atmosphere, or
air, completely surrounds the earth as a kind of envelope, and comes in
contact with everything upon its surface. It is composed chiefly of
oxygen and nitrogen,29 but it also contains a small per cent of other
substances, such as water-vapor, carbon dioxide, and argon. All of the
regular constituents of the atmosphere are gases, and these, as compared
with liquids and solids, are very light. Nevertheless the atmosphere has
weight and, on this account, exerts pressure upon everything on the
earth. At the sea level, its pressure is nearly fifteen pounds to the
square inch. The atmosphere forms an essential part of one’s physical
environment and serves various purposes. The process[pg 077] by which gaseous materials are
made to pass between the body and the atmosphere is known as

Respiration.—As usually defined,
respiration, or breathing, consists of two simple processes—that of
taking air into special contrivances in the body, called the lungs, and
that of expelling air from the lungs. The first process is known as inspiration; the second as expiration. We must, however, distinguish
between respiration by the lungs, called external respiration, and respiration by the cells, called
internal respiration.

The purpose of respiration is
indicated by the changes that take place in the air while it is in the
lungs. Air entering the lungs in ordinary breathing parts with about
five per cent of itself in the form of oxygen and receives about four
and one half per cent of carbon dioxide, considerable water-vapor, and a
small amount of other impurities. These changes suggest a twofold
purpose for respiration:

1. To obtain from the atmosphere the supply of oxygen needed by the
body.

2. To transfer to the atmosphere certain materials (wastes) which
must be removed from the body.

The chief organs concerned in the work of respiration are

The Lungs.—The lungs consist of two
sac-like bodies suspended in the thoracic cavity, and occupying all the
space not taken up by the heart. They are not simple sacs, however, but
are separated into numerous divisions, as follows:

1. The lung on the right side of the thorax, called the right lung,
is made up of three divisions, or lobes, and the left lung is made up of two lobes.

2. The lobes on either side are separated into smaller[pg 078] divisions, called lobules (Fig. 33). Each lobule receives a
distinct division of an air tube and has in itself the structure of a
miniature lung.

Fig. 33—Lungs and air
passages seen from the front. The right lung shows the lobes and
their divisions, the lobules. The tissue of the left lung has been
dissected away to show the air tubes.

3. In the lobule the air tube divides into a number of smaller tubes,
each ending in a thin-walled sac, called an infundibulum. The interior of the infundibulum is separated
into many small spaces, known as the alveoli, or air cells.

The lungs are remarkable for their lightness and delicacy of
structure.30 They consist chiefly of the tissues that form their sacs,
air tubes, and blood vessels; the membranes that line their inner and
outer surfaces; and the connective tissue that binds these parts
together. All these tissues are more or less elastic. The relation of
the different parts of the lungs to[pg 079] each other and to the outside
atmosphere will be seen through a study of the

Air Passages.—The air passages
consist of a system of tubes which form a continuous passageway between
the outside atmosphere and the different divisions of the lungs. The air
passes through them as it enters and leaves the lungs, a fact which
accounts for the name.

Fig. 34—Model of section
through the head, showing upper air passages and other parts. 1.
Left nostril. 2. Pharynx. 3. Tongue and cavity of mouth. 4. Larynx. 5.
Trachea. 6. Esophagus.

The incoming air first enters the nostrils. These consist of two narrow passages lying side
by side in the nose, and connecting with the pharynx behind. The lining
of the nostrils, called mucous
membrane is quite thick, and has its surface much extended by
reason of being spread over some thin, scroll-shaped bones that project
into the passage. This membrane is well supplied with blood vessels and
secretes a considerable quantity of liquid. Because of the nature and
arrangement of the membrane, the nostrils are able to warm and moisten the incoming air, and to free it from dust particles, preparing it, in this way, for
entrance into the lungs (Fig. 34).

The nostrils are separated from the mouth by a thin layer of bone,
and back of both the mouth and the nostrils is the pharynx. The pharynx and the mouth serve as parts of the food canal, as well as air
passages, and are[pg 080] described in connection with the
organs of digestion (Chapter X). Air entering the pharynx, either by the
nostrils or by the mouth, passes through it into the larynx. The larynx, being the special
organ for the production of the voice, is described later (Chapter XXI).
The entrance into the larynx is guarded by a movable lid of cartilage,
called the epiglottis, which prevents
food particles and liquids, on being swallowed, from passing into the
lower air tubes. The relations of the nostrils, mouth, pharynx, and
larynx are shown in Fig. 34.

From the larynx the air enters the trachea, or windpipe. This is a straight and nearly round
tube, slightly less than an inch in diameter and about four and one half
inches in length. Its walls contain from sixteen to twenty C-shaped,
cartilaginous rings, one above the other and encircling the tube. These
incomplete rings, with their openings directed backward, are held in
place by thin layers of connective and muscular tissue. At the lower end
the trachea divides into two branches, called the bronchi, each of which
closely resembles it in structure. Each bronchus separates into a number of smaller divisions,
called the bronchial tubes, and these
in turn divide into still smaller branches, known as the lesser bronchial tubes (Fig. 33). The
lesser bronchial tubes, and the branches into which they separate, are
the smallest of the air tubes. One of these joins, or expands into, each
of the minute lung sacs, or infundibula. Mucous membrane lines all of
the air passages.

General Condition of the Air
Passages.—One necessary condition for the movement of the air into
and from the lungs is an unobstructed passageway.31 The air
passages[pg 081] must be kept open and free from
obstructions. They are kept open by
special contrivances found in their walls, which, by supplying a degree
of stiffness, cause the tubes to keep their form. In the trachea,
bronchi, and larger bronchial tubes, the stiffness is supplied by rings
of cartilage, while in the smaller tubes this is replaced by connective
and muscular tissue. The walls of the larynx contain strips and plates
of cartilage; while the nostrils and the pharynx are kept open by their
bony surroundings.

Fig. 35—Ciliated
epithelial cells. A. Two cells
highly magnified. c. Cilia, n. Nucleus. B. Diagram of a small air tube showing the lining of
cilia.

The air passages are kept clean by
cells especially adapted to this purpose, known as the ciliated epithelial cells. These are
slender, wedge-shaped cells which have projecting from a free end many
small, hair-like bodies, called cilia
(Fig. 35). They line the mucous membrane in most of the air passages,
and are so placed that the cilia project into the tubes. Here they keep
up an inward and outward wave-like movement, which is quicker and has
greater force in the outward
direction. By this means the cilia are able to move small pieces of
foreign matter, such as dust particles and bits of partly dried mucus,
called phlegm, to places where they can be easily expelled from the
lungs.32

Fig. 36—Terminal air
sacs. The two large sacs are infundibula; the small divisions are
alveoli. (Enlarged.)

[pg 082]The
Alveoli.—The alveoli, or air cells, are the small divisions of the
infundibula (Fig. 36). They are each about one one-hundredth of an inch
(1/4 mm.) in diameter, being formed by the infolding of the infundibular
wall. This wall, which has for its framework a thin layer of elastic
connective tissue, supports a dense network of capillaries (Fig. 37),
and is lined by a single layer of cells placed edge to edge. By this
arrangement the air within the alveoli is brought very near a large
surface of blood, and the exchange of gases between the air and the
blood is made possible. It is at the alveoli that the oxygen passes from
the air into the blood, and the carbon dioxide passes from the blood
into the air. At no place in the lungs, however, do the air and the
blood come in direct contact. Their exchanges must in all cases take
place through the capillary walls and the layer of cells lining the
alveoli.

Fig. 37—Inner lung
surface (magnified), the blood vessels injected with coloring
matter. The small pits are alveoli, and the vessels in their walls are
chiefly capillaries.

Fig. 38.—Diagram to show the double movement of air and
blood through the lungs. The blood leaves the heart by the
pulmonary artery and returns by the pulmonary veins. The air enters and
leaves the lungs by the same system of tubes.

Fig. 39—Diagram to show
air and blood movements in a terminal air sac. While the air moves
into and from the space within the sac, the blood circulates through the
sac walls.

Blood Supply to the Lungs.—To
accomplish the purposes of respiration, not only the air, but the blood
also, must be passed into and from the lungs. The chief[pg 084] artery conveying blood to the
lungs is the pulmonary artery. This
starts at the right ventricle and by its branches conveys blood to the
capillaries surrounding the alveoli in all parts of the lungs. The
branches of the pulmonary artery lie alongside of, and divide similarly
to, the bronchial tubes. At the places where the finest divisions of the
air tubes enter the infundibula, the little arteries branch into the
capillaries that penetrate the infundibular walls (Figs. 38 and 39).
From these capillaries the blood is conveyed by the pulmonary veins to
the left auricle.

The lungs also receive blood from two (in some individuals three)
small arteries branching from the aorta, known as the bronchial arteries. These convey to the
lungs blood that has already been supplied with oxygen, passing it into
the capillaries in the walls of the bronchi, bronchial tubes, and large
blood vessels, as well as the connective tissue between the lobes of the
lungs. This blood leaves the lungs partly by the bronchial veins and
partly by the pulmonary veins. No part of the body is so well supplied
with blood as the lungs.

Fig. 40—The pleuræ.
Diagram showing the general form of the pleural sacs as they surround
the lungs and line the inner surfaces of the chest (other parts
removed). A, A’. Places occupied by
the lungs. B, B’. Slight space within
the pleural sacs containing the pleural secretion, a, a’. Outer layer of pleura and lining of chest walls and
upper surface of diaphragm. b, b’.
Inner layer of pleura and outer lining of lungs. C. Space occupied by the heart. D. Diaphragm.

The Pleura.—The pleura is a thin,
smooth, elastic, and tough membrane which covers the outside of the
lungs and lines the inside of the chest walls. The covering of each lung
is continuous with the lining of the chest wall on its respective side
and forms with it a closed sac by[pg 085] which the lung is surrounded, the
arrangement being similar to that of the pericardium. Properly speaking,
there are two pleuræ, one for each lung, and these, besides inclosing
the lungs, partition off a middle space which is occupied by the heart
(Fig. 40). They also cover the upper surface of the diaphragm, from
which they deflect upward, blending with the pericardium. A small amount
of liquid is secreted by the pleura, which prevents friction as the
surfaces glide over each other in breathing.

The Thorax.—The force required for
breathing is supplied by the box-like portion of the body in which the
lungs are placed. This is known as the thorax, or chest, and includes
that part of the trunk between the neck and the abdomen. The space which
it incloses, known as the thoracic cavity, is a variable space and the walls surrounding this space are air-tight. A framework for the thorax is
supplied by the ribs which connect with the spinal column behind and
with the sternum, or breast-bone, in front. They form joints with the
spinal column, but connect with the sternum by strips of cartilage. The
ribs do not encircle the cavity in a horizontal direction, but slope
downward from the spinal column both toward the front and toward the
sides, this being necessary to the service which they render in
breathing.

How Air is Brought into and Expelled
from the Lungs.—The principle involved in breathing is that air
flows from a place of greater to a
place of less pressure. The
construction of the thorax and the arrangement of the lungs within it
provide for the application of this principle in a most practical
manner. The lungs are suspended from the upper portion of the thoracic
cavity, and the trachea and the upper air passages provide the only
opening to the outside atmosphere. Air entering the thorax must on[pg 086] this account pass into the lungs.
As the thorax is enlarged the air in the lungs expands, and there is
produced within them a place of slightly
less air pressure than that of the atmosphere on the outside of the
body. This difference causes the air to flow into the lungs.

Fig. 41—Diagram illustrating the bellows principle in
breathing. A. The human bellows.
B. The hand bellows. Compare part for
part.

When the thorax is diminished in size, the air within the lungs is
slightly compressed. This causes it to become denser and to exert on
this account a pressure slightly
greater than that of the atmosphere on the outside. The air now
flows out until the equality of the pressure is again restored. Thus the
thorax, by making the pressure within the lungs first slightly less and
then slightly greater than the atmospheric pressure, causes the air to
move into and out of the lungs.

Breathing is well illustrated by means of the common hand bellows,
its action being similar to that of the thorax. It will be observed that
when the sides are spread apart air flows into the bellows. When they
are pressed together the air flows out. If an air-tight sack were hung
in the bellows with its mouth attached to the projecting tube, the
arrangement would resemble closely the general plan of the breathing
organs (Fig. 41). One respect, however, in which the bellows differs
from the thorax should be noted. The thorax is never sufficiently
compressed to drive out all the air. Air is always present in the lungs.
This keeps them more or less distended and pressed against the thoracic
walls.

How the Thoracic Space is
Varied.—One means of varying the size of the thoracic cavity is
through the movements of the ribs and their resultant effect upon the
walls[pg 087] of the thorax. In bringing about
these movements the following muscles are employed:

1. The scaleni muscles, three in
number on each side, which connect at one end with the vertebræ of the
neck and at the other with the first and second ribs. Their contraction
slightly raises the upper portion of the thorax.

2. The elevators of the ribs,
twelve in number on each side, which are so distributed that each single
muscle is attached, at one end, to the back portion of a rib and, at the
other, to a projection of the vertebra a few inches above. The effect of
their contraction is to’ elevate the middle portion of the ribs and to
turn them outward or spread them apart.

3. The intercostal muscles, which
form two thin layers between the ribs, known as the internal and the external intercostal muscles. The external intercostals are
attached between the outer lower margin of the rib above and the outer
upper margin of the rib below, and extend obliquely downward and
forward. The internal intercostals are attached between the inner
margins of adjacent ribs, and they extend obliquely downward and
backward from the front. The contraction of the external intercostal
muscles raises the ribs, and the contraction of the internal
intercostals tends to lower them.

Fig. 42—Simple
apparatus for illustrating effect of movements of the ribs upon the
thoracic space; strips of cardboard held together by pins, the front
part being raised or lowered by threads moving through attachments at 1
and 2. As the front is raised the space between the uprights is
increased. The front upright corresponds to the breastbone, the back one
to the spinal column, the connecting strips to the ribs, and the threads
to the intercostal muscles.

By slightly raising and spreading apart the ribs the thoracic space
is increased in two directions—from front to back and from side to side.
Lowering and converging the ribs has, of course, the opposite effect
(Fig. 42). Except in forced expirations the ribs are lowered and
converged by their own weight and by the elastic reaction of the
surrounding parts.

[pg 088]The Diaphragm.—Another means of
varying the thoracic space is found in an organ known as the diaphragm.
This is the dome-shaped, movable
partition which separates the thoracic cavity from the cavity of
the abdomen. The edges of the diaphragm are firmly attached to the walls
of the trunk, and the center is supported by the pericardium and the
pleura. The outer margin is muscular, but the central portion consists
of a strong sheet of connective tissue. By the contraction of its
muscles the diaphragm is pulled down, thereby increasing the thoracic
cavity. By raising the diaphragm the thoracic cavity is diminished.

The diaphragm, however, is not raised by the contraction of its own
muscles, but is pushed up by the
organs beneath. By the elastic reaction of the abdominal walls (after
their having been pushed out by the lowering of the diaphragm), pressure
is exerted on the organs of the abdomen and these in turn press against
the diaphragm. This crowds it into the thoracic space. In forced
expirations the muscles in the abdominal walls contract to push up the
diaphragm.

Interchange of Gases in the
Lungs.—During each inspiration the air from the outside fills the
entire system of bronchial tubes, but the alveoli are largely filled, at
the same time, by the air which the last expiratory effort has left in
the passages. By the action of currents and eddies and by the rapid
diffusion of gas particles, the air from the outside mixes with that in
the alveoli and comes in contact with the membranous walls. Here the
oxygen, after being dissolved by the moisture in the membrane, diffuses
into the blood. The carbon dioxide, on the other hand, being in excess
in the blood, diffuses toward the air in the alveoli. The interchange of
gases at the lungs, however, is not fully understood, and it is possible
that other forces than osmosis play a part.

Fig. 43—Diagram
illustrating lung capacity.

Capacity of the Lungs.—The air
which passes into and from the lungs in ordinary breathing, called the
tidal air, is but a small part of[pg 089] the whole amount of air which the
lungs contain. Even after a forced expiration the lungs are almost half
full; the air which remains is called the residual air. The air which is expelled from the lungs by a
forced expiration, less the tidal air, is called the reserve, or supplemental, air. These
several quantities are easily estimated. (See Practical Work.) In the
average individual the total capacity of the lungs (with the chest in
repose) is about one gallon. In forced inspirations this capacity may be
increased about one third, the excess being known as the complemental air (Fig. 43).

Fig. 44—Diagram
illustrating internal respiration and its dependence on external
respiration. (Modified from Hall.) (See text.)

Internal, or Cell, Respiration.—The
oxygen which enters the blood in the lungs leaves it in the tissues,
passing through the lymph into the cells (Fig. 44). At the same time the
carbon dioxide which is being formed at the cells passes into the blood.
An exchange of gases is thus taking place between the cells and the
blood, similar to[pg 090] that taking place between the
blood and the air. This exchange is known as internal, or cell, respiration. By internal respiration the
oxygen reaches the place where it is to serve its purpose, and the
carbon dioxide begins its movement toward the exterior of the body. This
“breathing by the cells” is, therefore, the final and essential act of respiration. Breathing by
the lungs is simply the means by which the taking up of oxygen and the giving off of carbon dioxide by the
cells is made possible.

HYGIENE OF RESPIRATORY ORGANS

The liability of the lungs to attacks from such dread diseases as
consumption and pneumonia makes questions touching their hygiene of
first importance. Consumption does not as a rule attack sound lung
tissue, but usually has its beginning in some weak or enfeebled spot in
the lungs which has lost its “power of resistance.” Though consumption
is not inherited, as some suppose, lung weaknesses may be transmitted
from parents to children. This, together with the fact, now generally
recognized, that consumption is contagious, accounts for the frequent
appearance of this disease in the same family. Consumption as well as
other respiratory affections can in the majority of cases be prevented, and in many cases cured, by an
intelligent observation of well-known laws of health.

Breathe through the Nostrils.—Pure
air and plenty of it is the main condition in the hygiene of the lungs.
One necessary provision for obtaining pure
air is that of breathing through the nostrils. Air is the carrier
of dust particles and not infrequently of disease germs.33 Partly
through[pg 091] the small hairs in the nose, but
mainly through the moist membrane that lines the passages, the nostrils
serve as filters for removing the minute solid particles (Fig. 45).
While it is important that nose breathing be observed at all times, it
is especially important when one is surrounded by a dusty or smoky
atmosphere. Otherwise the small particles that are breathed in through
the mouth may find a lodging place in the lungs.

Fig. 45—Human air
filter. Diagram of a section through the nostrils; shows projecting
bones covered with moist membrane against which the air is made to
strike by the narrow passages. 1. Air passages. 2. Cavities in the
bones. 3. Front lower portion of the cranial cavity.

In addition to removing dust particles and germs, other purposes are
served by breathing through the nostrils. The warmth and moisture which
the air receives in this way, prepare it for entering the lungs. Mouth
breathing, on the other hand, looks bad and during sleep causes snoring.
The habit of nose breathing should be established early in life.34

Cultivate Full Breathing.—Many
people, while apparently taking in sufficient air to supply their need
for oxygen, do not breathe deeply enough to “freely ventilate the
lungs.” “Shallow breathing,” as this is called,[pg 092] is objectionable because it fails
to keep up a healthy condition of the entire lung surface. Portions of
the lungs to which air does not easily penetrate fail to get the fresh
air and exercise which they need. As a consequence, they become weak
and, by losing their “power of resistance,” become points of attack in
diseases of the lungs.35 The breathing of each individual should
receive attention, and where from some cause it is not sufficiently full
and deep, the means should be found for remedying the defect.

Causes of Shallow
Breathing.—Anything that impedes the free movement of air into the
lungs tends to cause shallow breathing A drooping of the back or
shoulders and a curved condition of the spinal column, such as is caused
by an improper position in sitting, interfere with the free movements of
the ribs and are recognized causes. Clothing also may impede the
respiratory movements and lead to shallow breathing. If too tight around
the chest, clothing interferes with the elevation of the ribs; and if
too tight around the waist, it prevents the depression of the diaphragm.
Other causes of shallow breathing are found in the absence of vigorous
exercise, in the leading of an indoor and inactive life, in obstructions
in the nostrils and upper pharynx, and in the lack of attention to
proper methods of breathing.

To prevent shallow breathing one should have the habit of sitting and
standing erect. The clothing must not be allowed to interfere with the
respiratory movements. The taking of exercise sufficiently vigorous to
cause deep and[pg 093] rapid breathing should be a
common practice and one should spend considerable time out of doors. If
one has a flat chest or round shoulders, he should strive by suitable
exercises to overcome these defects. Obstructions in the nostrils or
pharynx should be removed.

Breathing Exercises.—In overcoming
the habit of shallow breathing and in strengthening the lungs generally,
the practicing of occasional deep breathing has been found most valuable
and is widely recommended. With the hands on the hips, the shoulders
drawn back and down, the chest pushed
upward and forward, and the chin slightly depressed, draw the air slowly
through the nostrils until the lungs are completely full. After holding this long enough to count
three slowly, expel it quickly from the lungs. Avoid straining. To get
the benefit of pure air, it is generally better to practice deep
breathing out of doors or before an open window.

By combining deep breathing with simple exercises of the arms,
shoulders, and trunk much may be done towards straightening the spine,
squaring the shoulders, and overcoming flatness of the chest. Though
such movements are best carried on by the aid of a physical director,
one can do much to help himself. One may safely proceed on the principle
that slight deformities of the chest, spine, and shoulders are corrected
by gaining and keeping the natural positions, and may employ any
movements which will loosen up the parts and bring them where they
naturally belong.36

[pg 094] Serious Nature of Colds.—That many cases of consumption have
their beginning in severe colds (on the lungs) is not only a matter of
popular belief, but the judgment also of physicians. Though the cold is
a different affection from that of consumption, it may so lower the
vitality of the body and weaken the lung surfaces that the germs of
consumption find it easy to get a start. On this account a cold on the
chest which does not disappear in a few days, but which persists,
causing more or less coughing and pain in the lungs, must be given
serious consideration.37 The usual home remedies failing to give
relief, a physician should be consulted. It should also be noted that
certain diseases of a serious nature (pneumonia, diphtheria, measles,
etc.) have in their beginning the appearance of colds. On this account
it is wise not only to call a physician, but to call him early, in
severe attacks of the lungs. Especially if the attack be attended by
difficult breathing, fever, and a rapid pulse is the case serious and
medical advice necessary.

Ventilation.—The process by which
the air in a room is kept fresh and pure is known as ventilation. It is
a[pg 095] double process—that of bringing
fresh air into the room and that of getting rid of air that has been
rendered impure by breathing 38 or by lamps. Outdoor air is usually of
a different temperature (colder in winter, warmer in summer) from that
indoors, and as a consequence differs from it slightly in weight. On
account of this difference, suitable openings in the walls of buildings
induce currents which pass between the rooms and the outside atmosphere
even when there is no wind. In winter care must be taken to prevent
drafts and to avoid too great a loss of heat from the room. A cold draft
may even cause more harm to one in delicate health than the breathing of
air which is impure. To ventilate a room successfully the problem of
preventing drafts must be considered along with that of admitting the
fresh air.

Fig. 46—Window adjusted for ventilation without
drafts.

The method of ventilation must also be adapted to the construction of
the building, the plan of heating, and the condition of the weather.
Specific directions cannot be given, but the following suggestions will
be found helpful in ventilating rooms where the air is not warmed before
being admitted:

1. Introduce, the air through many
small openings rather than a few large ones. If the windows are
used for this purpose, raise the lower sash and drop the upper one slightly for several windows, varying the width to suit the conditions
(Fig. 46). By this means sufficient air may be introduced without
causing drafts.

2. Introduce the air at the warmest
portions of the room.[pg 096] The air should, if possible, be
warmed before reaching the occupants.

3. If the wind is blowing, ventilate
principally on the sheltered side of the house.

Ample provision should be made for fresh air in sleeping rooms, and
here again drafts must be avoided. Especially should the bed be so
placed that strong air currents do not pass over the sleeper. In
schoolhouses and halls for public gatherings the means for efficient
ventilation should, if possible, be provided in the general plan of
construction and method of heating.

Fig. 47—Artificial respiration as a laboratory
experiment. Expiration. Prone-posture method of Schaffer.

Artificial Respiration.—When
natural breathing is temporarily suspended, as in partial drowning, or
when one has been overcome by breathing some poisonous gas, the saving
of life often depends upon the prompt application of artificial
respiration. This is accomplished by alternately compressing and
enlarging the thorax by means of variable pressure on the outside,
imitating the natural process as nearly as possible. Following is the
method proposed by Professor E.A. Schaffer of England, and called by
him “the prone-posture method of
artificial respiration”:

[pg 097]The patient is laid
face downward with an arm bent under the head, and intermittent pressure applied vertically over the shortest
ribs. The pressure drives the air from the lungs, both by compressing
the lower portions of the chest and by forcing the abdominal contents
against the diaphragm, while the elastic reaction of the parts causes
fresh air to enter (Figs. 47 and 48). “The operator kneels or squats by
the side of, or across the patient, places his hands over the lowest
ribs and swings his body backward and forward so as to allow his weight
to fall vertically on the wrists and then to be removed; in this way
hardly any muscular exertion is required…. The pressure is applied
gradually and slowly, occupying some three seconds; it is then withdrawn
during two seconds and again applied; and so on some twelve times per
minute.”39

Fig. 48—Artificial respiration. Inspiration.

The special advantages of the prone-posture method over others that
have been employed are: I. It may be applied by a single individual and
fora long period of time without exhaustion. 2. It allows the mucus and
water (in case of drowning) to run out of the mouth, and causes the
tongue to fall forward so as not to obstruct the passageway. 3. It
brings a sufficient amount of air into the lungs.40

[pg 098]While applying artificial
respiration, the heat of the body should not be allowed to escape any
more than can possibly be helped. In case of drowning, the patient
should be wrapped in dry blankets or clothing, while bottles of hot
water may be placed in contact with the body. The circulation should be
stimulated, as may be done by rubbing the hands, feet, or limbs in the
direction of the flow of the blood in the veins.

Tobacco Smoke and the Air
Passages.—Smoke consists of minute particles of unburnt carbon, or soot,
such as collect in the chimneys of fireplaces and furnaces. If much
smoke is taken into the lungs, it irritates the delicate linings and
tends to clog them up. Tobacco smoke also contains the poison nicotine,
which is absorbed into the blood. For these reasons the cigarette user
who inhales the smoke does himself great harm, injuring his nervous
system and laying the foundation for diseases of the air passages. The
practice of smoking indoors is likewise objectionable, since every one
in a room containing the smoke is compelled to breathe it.

Alcohol and Diseases of the
Lungs.—Pneumonia is a serious disease of the lungs caused by germs.
The attacks occur as a result of exposure, especially when the body is
in a weakened condition. A noted authority states that “alcoholism is
perhaps the most potent predisposing cause” of pneumonia.41 A person
addicted to the use of alcohol is also less likely to recover from the
disease than one who has avoided its use, a result due in part to the
weakening effect of alcohol upon the heart. The congestion of the lungs
in pneumonia makes it very difficult for the heart to force the blood
through them. The weakened heart of the drunkard gives way under the
task.

The statement sometimes made that alcohol is beneficial[pg 099] in pulmonary tuberculosis is
without foundation in fact. On the other hand, alcoholism is a
recognized cause of consumption. Some authorities claim that this
disease is more frequent in heavy drinkers than in those of temperate
habits, in the proportion of about three to one, and that possibly half
of the cases of tuberculosis are traceable to alcoholism.42

The Outdoor Cure for Lung
Diseases—Among the many remedies proposed for consumption and
kindred diseases, none have proved more beneficial, according to
reports, than the so-called “outdoor” cure. The person having
consumption is fed plentifully upon the most nourishing food, and is
made to spend practically his entire time, including the sleeping hours,
out of doors. Not only is this done
during the pleasant months of summer, but also during the winter when
the temperature is below freezing. Severe exposure is prevented by
overhead protection at night and by sufficient clothing to keep the body
warm. The abundant supply of pure, cold air toughens the lungs and
invigorates the entire body, thereby enabling it to throw off the
disease.

The success attending this method of treating consumptives suggests
the proper mode of strengthening lungs that are not diseased, but simply
weak. The person having weak lungs should spend as much time as he
conveniently can out of doors. He should provide the most ample
ventilation at night and have a sleeping room to himself. He should
practice deep breathing exercises and partake of a nourishing diet.
While avoiding prolonged chilling and other conditions liable to induce
colds, he should take advantage of every opportunity of exposing himself
fully and freely to the outside atmosphere.

Summary.—The purpose of respiration
is to bring about an exchange of gases between the body and the
atmosphere. The organs employed for this purpose, called the respiratory
organs, are adapted to handling materials in the gaseous state, and are operated in accordance with
principles governing the movements of the atmosphere. By alternately
increasing and diminishing[pg 100] the thoracic space, air is made
to pass between the outside atmosphere and the interior of the lungs.
Finding its way into the smallest divisions of the lungs, called the
alveoli, the air comes very near a large surface of blood. By this means
the carbon dioxide diffuses out of the blood, and the free oxygen
enters. Through the combined action of the organs of respiration and the
organs that move the blood and the lymph, the cells in all parts of the
body are enabled to exchange certain gaseous materials with the outside
atmosphere.

Fig. 49—Model for demonstrating the lungs.

Exercises.—1. How does air entering
the lungs differ in composition from air leaving the lungs? What
purposes of respiration are indicated by these differences?

2. Name the divisions of the lungs.

3. Trace air from the outside atmosphere into the alveoli. Trace the
blood from the right ventricle to the alveoli and back again to the left
auricle.

4. How does the movement of air into and from the lungs differ from
that of the blood through the lungs with respect to (a) the direction of the motion. (b) the causes of the motion, and (c) the tubes through which the motion
takes place?

5. How are the air passages kept clean and open?

6. Describe the pleura. Into what divisions does it separate the
thoracic cavity?

7. Describe and name uses of the diaphragm.

8. If 30 cubic inches of air are passed into the lungs at each
inspiration and .05 of this is retained as oxygen, calculate the number
of cubic feet of oxygen consumed each day, if the number of inspirations
be 18 per minute.

9. Find the weight of a day’s
supply of oxygen, as found in the above problem, allowing 1.3 ounces as
the weight of a cubic foot.

10. Make a study of the hygienic ventilation of the schoolroom.

[pg 101]11. Give advantages of full breathing over shallow breathing.

12. How may a flat chest and round shoulders be a cause of
consumption? How may these deformities be corrected?

13. Give general directions for applying artificial respiration.

PRACTICAL WORK

Examine a dissectible model of the chest and its contents (Fig. 49).
Note the relative size of the two lungs and their position with
reference to the heart and diaphragm. Compare the side to side and
vertical diameters of the cavity. Trace the air tubes from the trachea
to their smallest divisions.

Observation of Lungs
(Optional).—Secure from a butcher the lungs of a sheep, calf, or hog.
The windpipe and heart should be left attached and the specimen kept in
a moist condition until used. Demonstrate the trachea, bronchi, and the
bronchial tubes, and the general arrangement of pulmonary arteries and
veins. Examine the pleura and show lightness of lung tissue by floating
a piece on water.

To show the Changes that Air undergoes
in the Lungs.—1. Fill a quart jar even full of water. Place a piece
of cardboard over its mouth and invert, without spilling, in a pan of
water. Inserting a tube under the jar, blow into it air that has been
held as long as possible in the lungs. When filled with air, remove the
jar from the pan, keeping the top well covered. Slipping the cover
slightly to one side, insert a burning splinter and observe that the
flame is extinguished. This proves the absence of sufficient oxygen to
support combustion. Pour in a little limewater43 and shake to mix with
the air. The change of the limewater to a milky white color proves the
presence of carbon dioxide.

Fig. 50—Apparatus
for showing changes which air undergoes while in the lungs.

2. The effects illustrated in experiment 1 may be shown in a somewhat
more striking manner as follows: Fill two bottles of the same[pg 102] size each one fourth full of
limewater and fit each with a two-holed rubber stopper (Fig. 50). Fit
into each stopper one short and one long glass tube, the long tube
extending below the limewater. Connect the short tube of one bottle and
the long tube of the other bottle with a Y-tube. Now breathe slowly
three or four times through the Y-tube. It will be found that the
inspired air passes through one bottle and the expired air through the
other. Compare the effect upon the limewater in the two bottles. Insert
a small burning splinter into the top of each bottle and note result.
What differences between inspired and expired air are thus shown?

3. Blow the breath against a cold window pane. Note and account for
the collection of moisture.

4. Note the temperature of the room as shown by a thermometer. Now
breathe several times upon the bulb, noting the rise in the mercury.
What does this experiment show the body to be losing through the
breath?

To show Changes in the Thoracic
Cavity.—1. To a yard- or meter-stick, attach two vertical strips,
each about eight inches long, as shown in Fig. 51. The piece at the end
should be secured firmly in place by screws or nails. The other should
be movable. With this contrivance measure the sideward and forward
expansion of a boy’s thorax. Take the diameter first during a complete
inspiration and then during a complete expiration, reading the
difference. Compare the forward with the sideward expansion.

Fig. 51—Apparatus
for measuring chest expansion.

2. With a tape-line take the circumference of the chest when all the
air possible has been expelled from the lungs. Take it again when the
lungs have been fully inflated. The difference is now read as the chest
expansion.

Fig. 52—Simple
apparatus for illustrating the action of the diaphragm.

To illustrate the Action of the
Diaphragm.—Remove the bottom from a large bottle having a small
neck. (Scratch a deep mark with a[pg 103] file and hold on the end of this
mark a hot poker. When the glass cracks, lead the crack around the
bottle by heating about one half inch in advance of it.) Place the
bottle in a large glass jar filled two thirds full of water (Fig. 52).
Let the space above the water represent the chest cavity and the water
surface represent the diaphragm. Raise the bottle, noting that the water
falls, thereby increasing the space and causing air to enter. Then lower
the bottle, noting the opposite effect. To show the movement of the air
in and out of the bottle, hold with the hand (or arrange a support for)
a burning splinter over the mouth of the bottle.

To estimate the Capacity of the
Lungs.—Breathing as naturally as possible, expel the air into a
spirometer (lung tester) during a period, say of ten respirations (Fig.
53). Note the total amount of air exhaled and the number of “breaths”
and calculate the amount of air exhaled at each breath. This is called
the tidal air.

Fig. 53—Apparatus
(spirometer) for measuring the capacity of the lungs.

2. After an ordinary inspiration empty the lungs as completely as
possible into the spirometer, noting the quantity exhaled. This amount,
less the tidal air, is known as the reserve air. The air which is now left in the lungs is
called the residual air. On the
theory that this is equal in amount to the reserve air, calculate the
capacity of the lungs in an ordinary inspiration.

3. Now fill the lungs to the full expansion of the chest and empty
them as completely as possible into the spirometer, noting the amount
expelled. This, less the tidal air and the reserve air, is called the
complemental air. Now calculate the
total capacity of the lungs.

CHAPTER VIII – PASSAGE OF OXYGEN THROUGH THE BODY

What is the nature of oxygen? What is its purpose in the body and how
does it serve this purpose? How is the blood able to take it up at the
lungs and give it off at the cells? What becomes of it after being used?
These are questions touching the maintenance of life and they deserve
careful consideration.

Nature of Oxygen.—To understand the
relation which oxygen sustains to the body we must acquaint ourselves
with certain of its chemical properties. It is an element44 of intense
affinity, or combining power, and is one of the most active of all
chemical agents. It is able to combine with most of the other elements
to form chemical compounds. A familiar example of its combining action
is found in ordinary combustion, or burning. On account of the part it
plays in this process, oxygen is called the supporter of combustion; but it supports combustion by the
simple method of uniting. The ashes that are left and the invisible
gases that escape into the atmosphere are the compounds formed by the
uniting process. It thus appears that oxygen, in common with the other
elements, may exist in either of two forms:

[pg 105]1. That in which it is in a free,
or uncombined, condition—the form in which it exists in the
atmosphere.

2. That in which it is a part of compounds, such as the compounds
formed in combustion.

Oxygen manifests its activity to the best advantage when it is in a
free state, or, more accurately speaking, when it is passing from the
free state into one of combination. It is separated from its compounds
and brought again into a free state by overcoming with heat, or some
other force, the affinity which causes it to unite.

How Oxygen unites.—The chemist
believes oxygen, as well as all other substances, to be made up of
exceedingly small particles, called atoms. The atoms do not exist singly in either elements or
compounds, but are united with each other to form groups of atoms that
are called molecules. In an element
the molecules are made up of one kind of atoms, but in a compound the
molecules are made up of as many kinds of atoms as there are elements in
the compound. Changes in the composition of substances (called chemical
changes) are due to rearrangements of the atoms and the formation of new
molecules. The atoms, therefore, are the units of chemical combination.
In the formation of new compounds they unite, and in the breaking up of
existing compounds they separate.

The uniting of oxygen is no exception to this general law. All of its
combinations are brought about by the uniting of its atoms. In the
burning of carbon, for example, the atoms of oxygen and the atoms of
carbon unite, forming molecules of the compound known as carbon dioxide.
The chemical formula of this compound, which is CO_2, shows the
proportion in which the atoms unite—one atom of carbon uniting with two
atoms of oxygen in each of the molecules. The affinity of oxygen for
other[pg 106] elements, and the affinity of
other elements for oxygen, and for each other, resides in their
atoms.

Oxidation.—The uniting of oxygen
with other elements is termed oxidation. This may take place slowly or rapidly, the two
rates being designated as slow
oxidation and rapid oxidation.
Examples of slow oxidation are found in certain kinds of decay and in
the rusting of iron. Combustion is an example of rapid oxidation. Slow
and rapid oxidation, while differing widely in their effects upon
surrounding objects, are alike in that both produce heat and form
compounds of oxygen. In slow oxidation, however, the heat may come off
so gradually that it is not observed.

Movement of Oxygen through the
Body.—Oxygen has been shown in the preceding chapters to pass from
the lungs into the blood and later to leave the blood and, passing
through the lymph, to enter the cells. That oxygen does not become a
permanent constituent of the cells is shown by the constancy of the body
weight. Nearly two pounds of oxygen per day are known to enter the cells
of the average-sized person. If this became a permanent part of the
cells, the body would increase in weight from day to day. Since the body
weight remains constant, or nearly so, we must conclude that oxygen
leaves the body about as fast as it enters. Oxygen enters the body as a
free element. The form in which it
leaves the body will be understood when we realize the purpose which it
serves and the method by which it serves this purpose.

Purpose of Oxygen in the Body.—The
question may be raised: Is it possible for oxygen to serve a purpose in
the body without remaining in it? This, of course, depends upon what the
purpose is. That it is possible for oxygen to serve a purpose and at the
same time pass on through[pg 107] the place where it serves that
purpose, is seen by studying the combustion in an ordinary stove (Fig.
54). Oxygen enters at the draft and for the most part passes out at the
flue, but in passing through the stove it unites with, or oxidizes, the
fuel, causing the combustion which produces the heat.

Fig. 54—Coal stove
illustrating rapid oxidation.

Now it is found that certain chemical processes, mainly oxidations,
are taking place in the body. These produce the heat for keeping it warm
and also supply other forms of energy,45 including motion. It is the
purpose of oxygen to keep up these oxidations and, by so doing, to aid
in supplying the body with energy. It serves this purpose in much the
same way that it supports combustion, i.e., by uniting with, or oxidizing, materials derived from
foods that are present in the cells.

Does Oxygen serve Other
Purposes?—It has been suggested that oxygen may serve the purpose
of oxidizing, or destroying, substances that are injurious and of
acting, in this way, as a purifying agent in the body. In support of
this view is the natural tendency of oxygen to unite with substances and
the well-known fact that oxygen is an important natural agent in
purifying water. It seems probable, therefore, that it may to a slight
extent serve this purpose in the body. It is probable also that oxygen
aids through its chemical activity in the formation of compounds[pg 108] which are to become a part of the
cells. Both of these uses, however, are of minor importance when
compared with the main use of oxygen,
which is that of an aid in supplying
energy to the body.

Oxygen and the Maintenance of
Life.—In the supplying of energy to the body, one of the conditions
necessary to the maintenance of life is provided. Because oxygen is
necessary to this process, and because death quickly results when the
supply of it is cut off, oxygen is frequently called the supporter of
life. This idea is misleading, for oxygen has no more to do with the
maintenance of life than have the food materials with which it unites.
Life appears to be more dependent upon oxygen than upon food, simply
because the supply of it in the body at any time is exceedingly small.
Being continually surrounded by an atmosphere containing free oxygen,
the body depends upon this as a constant source of supply, and does not
store it up. Food, on the other hand, is taken in excess of the body’s
needs and stored in the various tissues, the supply being sufficient to
last for several days. When the supply of either oxygen or food is
exhausted in the body, life must cease.

The Oxygen Movement a
Necessity.—Since free oxygen is
required for keeping up the chemical changes in the cells, and since it
ceases to be free as soon as it goes into combination, its continuous
movement through the body is a necessity. The oxygen compounds must be
removed as fast as formed in order to make room for more free oxygen.
This movement has already been studied in connection with the blood and
the organs of respiration, but the consideration of certain details has
been deferred till now. By what means and in what form is the oxygen
passed to and from the cells?

[pg 109]Passage of Oxygen through the
Blood.—In serving its purpose at the cells, the oxygen passes twice
through the blood—once as it goes toward the cells and again as it
passes from the cells to the exterior of the body:

Passage toward the Cells.—This is
effected mainly through the hemoglobin of the red corpuscles. At the
lungs the oxygen and the hemoglobin form a weak chemical compound that
breaks up and liberates the oxygen when it reaches the capillaries in
the tissues. The separation of the oxygen from the hemoglobin at the
tissues appears to be due to two causes: first, to the weakness of the
chemical attraction between the atoms of oxygen and the atoms that make
up the hemoglobin molecule; and second, to a difference in the so-called
oxygen pressure at the lungs and at
the tissues.46

The attraction of the oxygen and the hemoglobin is sufficient to
cause them to unite where the oxygen pressure is more than one half
pound to the square inch, but it is not sufficiently strong to cause
them to unite or to prevent their separation, if already united, where
the oxygen pressure is less than one half pound to the square inch. The
oxygen pressure at the lungs, which amounts to nearly three pounds to
the square inch, easily causes the oxygen and the hemoglobin to unite,
while the almost complete absence of any oxygen pressure at the tissues,
permits their separation. The blood in its circulation constantly flows
from the place of high oxygen pressure at the lungs[pg 110] to the place of low oxygen
pressure at the tissues and, in so doing, loads up with oxygen at one
place and unloads it at the other (Fig. 55).

Passage from the Cells.—Since
oxygen leaves the free state at the cells and becomes a part of
compounds, we are able to trace it from the body only by following the
course of these compounds. Three waste compounds of importance are
formed at the cells—carbon dioxide (CO2), water (H2O), and urea
(N2H4CO). The first is formed by the union of oxygen with carbon,
the second by its union with hydrogen, and the third by its union with
nitrogen, hydrogen, and carbon. These compounds are carried by the blood
to the organs of excretion, where they are removed from the body. The
water leaves the body chiefly as a liquid, the urea as a solid dissolved
in water, and the carbon dioxide as a gas. The passage of carbon dioxide
through the blood requires special consideration.

Fig. 55—Diagram
illustrating movement, of oxygen and carbon dioxide through the
body (S.D. Magers). Each moves from a place of relatively high to a
place of relatively low pressure. (See text.)

Passage of Carbon Dioxide through the
Blood.—Part of the carbon dioxide is dissolved in the plasma of the
blood, and part of it is in weak chemical combination with substances
found in the plasma and in the corpuscles. Its passage through the blood
is accounted for in the same[pg 111] way as the passage of the oxygen.
Its ability to dissolve in liquids and to enter into chemical
combination varies as the carbon dioxide
pressure47 This in turn varies with the amount of the carbon
dioxide, which is greatest at the cells (where it is formed), less in
the blood, and still less in the lungs. Because of these differences,
the blood is able to take it up at the cells and release it at the lungs
(Fig. 55).

Fig. 56—Soap bubble
floating in a vessel of carbon dioxide, illustrating the difference in
weight between air and carbon dioxide gas.

Properties of Carbon
Dioxide.—Carbon dioxide is a colorless gas with little or no odor.
It is classed as a heavy gas, being about one third heavier than air48
(Fig. 56). It does not support combustion, but on the contrary is used
to some extent to extinguish fires. It is formed by the oxidation of
carbon in the body, and by the combustion of carbon outside of the body.
It is also formed by the decay of animal and vegetable matter. From
these sources it is continually finding its way into the atmosphere.
Although not a poisonous gas, carbon dioxide may, if it surround the
body, shut out the supply of oxygen and cause death.49

[pg 112]Final Disposition of Carbon
Dioxide.—It is readily seen that the union of carbon and oxygen,
which is continually removing oxygen from the air and replacing it with
carbon dioxide, tends to make the whole atmosphere deficient in the one
and to have an excess of the other. This tendency is counteracted
through the agency of vegetation. Green plants absorb the carbon dioxide
from the air, decompose it, build the carbon into compounds (starch,
etc.) that become a part of the plant, and return the free oxygen to the
air (Fig. 57). In doing this, they not only preserve the necessary
proportion of oxygen and carbon dioxide in the atmosphere, but also put
the carbon and oxygen in such a condition that they can again unite. The
force which enables the plant cells to decompose the carbon dioxide is
supplied by the sunlight (Chapter XII).

Fig. 57—Under
surface of a geranium leaf showing breathing pores, highly
magnified (O.H.).

Summary.—Oxygen, by uniting with
materials at the cells, keeps up a condition of chemical activity
(oxidation) in the body. This supplies heat and the other forms of
bodily energy. Entering as a free element, oxygen leaves the body as a
part of the waste compounds which it helps to form. The free oxygen is
transported from the lungs to the cells by means of the hemoglobin of
the red corpuscles, while the combined oxygen in carbon dioxide and
other compounds from the cells is carried mainly by the plasma. The
limited supply of free oxygen in the body at any time makes necessary
its continuous introduction into the body.

[pg 113]Exercises.—1. Describe the
properties of oxygen. How does it unite with other elements? How does it
support combustion?

2. State the purpose of oxygen in the body. What properties enable it
to fulfill this purpose?

3. What is the proof that oxygen does not remain permanently in the
body? How does the oxygen entering the body differ from the same oxygen
as it leaves the body?

4. What is the necessity for the continuous introduction of oxygen into the body, while food
is introduced only at intervals?

5. How are the red corpuscles able to take up and give off oxygen?
How is the plasma able to take up and give off carbon dioxide?

6. If thirty cubic inches of air pass from the lungs at each
expiration and 4.5 per cent of this is carbon dioxide, calculate the
number of cubic feet of the gas expelled in twenty-four hours,
estimating the number of respirations at eighteen per minute.

7. What is the weight of this volume of carbon dioxide, if one cubic
foot weigh 1.79 ounces?

8. What portion of this weight is oxygen and what carbon, the ratio
by weight of carbon to oxygen in carbon dioxide being twelve to
thirty-two?

9. What is the final disposition of carbon dioxide in the
atmosphere?

PRACTICAL WORK

To show the Difference between Free
Oxygen and Oxygen in Combination.—Examine some crystals of
potassium chlorate (KClO3). They contain oxygen in combination with potassium and chlorine. Place a few of
these in a small test tube and heat strongly in a gas or alcohol flame.
The crystals first melt, and the liquid which they form soon appears to
boil. If a splinter, having a spark on the end, is now inserted in the
tube, it is kindled into a flame. This shows the presence of free oxygen, the heat having caused the
potassium chlorate to decompose. The difference between free and
combined oxygen may also be shown by decomposing other compounds of
oxygen, such as water and mercuric oxide.

Preparation and Properties of
Oxygen.—Intimately mix 3 grams (1/2 teaspoonful) of potassium
chlorate with half its bulk of manganese dioxide, and place the mixture
in a large test tube. Close the test tube with a tight-fitting stopper
which bears a glass tube of sufficient[pg 114] length and of the right shape to convey the escaping gas to a small
trough or pan partly filled with water, on the table. Fill four
large-mouthed bottles with water and, by covering with cardboard, invert
each in the trough of water. Arrange the test tube conveniently for
heating, letting the end of the glass tube terminate under the mouth of
one of the bottles (Fig. 58). Using an alcohol lamp or a Bunsen burner,
heat over the greater portion of the tube at first, but gradually
concentrate the flame upon the mixture. Do not heat too strongly, and
when the gas is coming off rapidly, remove the flame entirely, putting
it back as the action slows down. After all the bottles have been
filled, remove the end of the glass tube from the water, but leave the
bottles of oxygen inverted in the trough until they are to be used. On
removing the bottles from the trough, keep the tops covered with wet
cardboard.

Fig. 58—Apparatus
for generating oxygen.

1. Examine a bottle of oxygen, noting its lack of color. Insert a
small burning splinter in the upper part of the bottle and observe the
change in the rate of burning. The air contains free oxygen, but it is
diluted with nitrogen. Compare this with the undiluted oxygen in the
bottle as to effect in causing the splinter to burn.

2. In a second bottle of oxygen insert a splinter without the flame,
but having a small spark on the end. As soon as the oxygen kindles the
spark into a flame, withdraw from the bottle and blow out the flame, but
again insert the spark. Repeat the experiment as long as the spark is
kindled by the oxygen into a flame. This experiment is usually performed
as a test for undiluted oxygen.

3. Make a hollow cavity in the end of a short piece of crayon. Fasten
a wire to the crayon, and fill the cavity with powdered sulphur.[pg 115] Ignite the sulphur in the flame
of an alcohol lamp or Bunsen burner, and lower it into a bottle of
oxygen. Observe the change in the rate of burning, the color of the
flame, and the material formed in the bottle by the burning. The gas
remaining in the bottle is sulphur dioxide (SO2), formed by the uniting of the sulphur and the
oxygen.

4. Bend a small loop on the end of a piece of picture wire. Heat the
loop in a flame and insert it in some powdered sulphur. Ignite the
melted sulphur which adheres, and insert it quickly in a bottle of
oxygen. Observe the dark, brittle material which is formed by the
burning of the iron. It is a compound of the iron with oxygen, similar
to iron rust, and formed by their uniting.

Preparation and Properties of Carbon
Dioxide.—1. (a) Attach a piece
of carbon (charcoal) no larger than the end of the thumb to a piece of
wire. Ignite the charcoal in a hot flame and lower it into a vessel of
oxygen. Observe its combustion, letting it remain in the bottle until it
ceases to burn. Note that the burning has consumed a part of the carbon
and has used up the free oxygen. Has anything been formed in their
stead?

(b) Remove the charcoal and add a
little limewater. Cover the bottle with a piece of cardboard, and bring
the gas and the limewater in contact by shaking. Note any change in the
color of the limewater. If it turns white, the presence of carbon
dioxide is proved.

2. Burn a splinter in a large vessel of air, keeping the top covered.
Add limewater and shake. Note and account for the result.

3. Place several pieces of marble (limestone) in a jar holding at
least half a gallon. Barely cover the marble with water, and then add
hydrochloric acid until a gas is rapidly evolved. This gas is carbon
dioxide.

(a) Does it possess color?

(b) Insert a burning splinter to
see if it supports combustion.

(c) Place a bottle of oxygen by
the side of the vessel of carbon dioxide. Light a splinter and
extinguish the flame by lowering it into the vessel of carbon dioxide.
Withdraw immediately, and if a spark remains on the splinter, thrust it
into the bottle of oxygen. Then insert the relighted splinter into the
carbon dioxide. Repeat several times, kindling the flame in one gas and
extinguishing it in the other. Finally show that the spark also may be
extinguished by holding the splinter a little longer in the carbon
dioxide.

(d) Tip the jar containing the
carbon dioxide over the mouth of a tumbler, as in pouring water, though
not far enough to spill the acid, and[pg 116] then insert a burning splinter in
the tumbler. Account for the result. Inference as to the weight of
carbon dioxide.

Fig. 59—Simple
apparatus for illustrating passage of oxygen through the body.

(e) Review experiments (page 101)
showing the presence of carbon dioxide in the breath.

To illustrate the General Movement of
Oxygen through the Body.—Into a glass tube, six inches in length
and open at both ends, place several small lumps of charcoal (Fig. 59).
Fit into one end of this tube, by means of a stopper, a smaller glass
tube which is bent at right angles and which is made to pass through a
close-fitting stopper to the bottom of a small bottle. Another small
tube is fitted into a second hole in this stopper, but terminating near
the top of the bottle, and to this is connected a rubber tube about
eighteen inches in length. The arrangement is now such that by sucking
air from the top of the bottle, it is made to enter at the distant end
of the tube containing the charcoal. After filling the bottle one third
full of limewater, heat the tube containing the charcoal until it begins
to glow. Then suck the air through the apparatus (as in smoking, without
drawing it into the lungs), observing what happens both in the tube and
in the bottle. What are the proofs that the oxygen, in passing through
the tube, unites with the carbon, forms carbon dioxide, and liberates
energy? Compare the changes which the oxygen undergoes while passing
through the tube with the changes which it undergoes in passing through
the body.

CHAPTER IX – FOODS AND THE THEORY OF DIGESTION

The body is constantly in need of new material. Oxidation, as shown
in the preceding chapter, rapidly destroys substances at the cells, and
these have to be replaced. Upon this renewal depends the supply of
energy. Moreover, there is found to be an actual breaking down of the
living material, or protoplasm, in the body. While this does not destroy
the cells, as is sometimes erroneously stated, it reduces the quantity
of the protoplasm and makes necessary a process of repair, or
rebuilding, of the tissues. This also requires new material. Finally,
substances, such as water and common salt, are required for the aid
which they render in the general work of the body. Since these are
constantly being lost in one way or another, they also must be replaced.
These different needs of the body for new materials are supplied
through

The Foods.—Foods are substances
that, on being taken into the healthy body, are of assistance in
carrying on its work. This definition properly includes oxygen, but the
term is usually limited to substances introduced through the digestive
organs. As suggested above, foods serve at least three purposes:

1. They, with oxygen, supply the body with energy.

2. They provide materials for rebuilding the tissues.

3. They supply materials that aid directly or indirectly in the
general work of the body.

[pg 118]The
Simple Foods, or Nutrients.—From the great variety of things that
are eaten, it might appear that many different kinds of substances are
suitable for food. When our various animal and vegetable foods are
analyzed, however, they are found to be similar in composition and to
contain only some five or six kinds of materials that are essentially
different. While certain foods may contain only a single one of these,
most of the foods are mixtures of two or more. These few common
materials which, in different proportions, form the different things
that are eaten, are variously referred to as simple foods, food-stuffs,
and nutrients, the last name being
the one generally preferred. The different classes of nutrients are as
follows:

It is now necessary to become somewhat familiar with the different
nutrients and the purposes which they serve in the body.

Proteids.—The proteids are obtained
in part from the animal and in part from the plant kingdom, there being
several varieties. A well-known variety, called albumin, is found in the white of eggs and in the plasma of
the blood, while the muscles contain an abundance of another variety,
known as myosin. Cheese consists
largely of a kind of proteid, called casein, which is also present in milk, but in a more
diluted form. If a mouthful of wheat is chewed for some time, most of it
is dissolved and swallowed, but there remains in the mouth a sticky,
gum-like substance. This is gluten, a
form of proteid which occurs[pg 119] in different grains. Again,
certain vegetables, as beans, peas, and peanuts, are rich in a kind of
proteid which is called legumen.

Proteids are compounds of carbon, hydrogen, oxygen, nitrogen, and a
small per cent of sulphur. Certain ones (the nucleo-proteids from
grains) also contain phosphorus. All of the proteids are highly complex
compounds and form a most important class of nutrients.

Purposes of Proteids.—The chief
purpose of proteids in the body is to rebuild the tissues. Not only do
they supply all of the main elements in the tissues, but they are of
such a nature chemically that they are readily built into the
protoplasm. They are absolutely essential to life, no other nutrients
being able to take their place. An animal deprived of them exhausts the
proteids in its body and then dies. In addition to rebuilding the
tissues, proteids may also be oxidized to supply the body with
energy.

Albuminoids form a small class of
foods, of minor importance, which are similar to proteids in
composition, but differ from them in being unable to rebuild the
tissues. Gelatin, a constituent of soup and obtained from bones and
connective tissue by boiling, is the best known of the albuminoid foods.
On account of the nitrogen which they contain, proteids and albuminoids
are often classed together as nitrogenous
foods.

Carbohydrates.—While the
carbohydrates are not so essential to life as are the proteids, they are
of very great value in the body. They are composed of carbon, hydrogen,
and oxygen, and are obtained mainly from plants. There are several
varieties of carbohydrates, but they are similar in composition. All of
those used as food to any great extent are starch and certain kinds of
sugar.

[pg 120]Starch is the carbohydrate of greatest importance as a food,
and it is also the one found in the greatest abundance. All green plants
form more or less starch, and many of them store it in their leaves,
seeds, or roots (Fig. 60). From these sources it is obtained as food.
Glycogen, a substance closely
resembling starch, is found in the body of the oyster. It is also formed
in the liver and muscles of the higher animals, being prepared from the
sugar of the blood, and is stored by them as reserve food (Chapter XI).
Glycogen is, on this account, called animal starch. Starch on being eaten is first changed to
sugar, after which it may be converted into glycogen in the liver and in
the muscles.

Fig. 60—Starch
grains in cells of potato as they appear under the microscope. (See
practical work.)

Sugars.—There are several varieties
of sugar, but the important ones used as foods fall into one or the
other of two classes, known as double
sugars (disaccharides) and single
sugars (monosaccharides). To the first class belong cane sugar, found in sugar cane and
beets, milk sugar, found in sweet
milk, and maltose, a kind of sugar
which is made from starch by the action of malt. The important members
of the second class are grape sugar,
or dextrose, and fruit sugar, or
levulose, both of which are found in fruits and in honey.

The most important of all sugars, so far as its use in the body is
concerned, is dextrose. To this form
all the other sugars, and starch also, are converted before they are
finally used in the body. The close chemical relation[pg 121] between the different
carbohydrates makes such a conversion easily possible.

Fats.—The fats used as foods belong
to one or the other of two classes, known as solid fats and oils. The
solid fats are derived chiefly from animals, and the oils are obtained
mostly from plants. Butter, the fat of meats, olive oil, and the oil of
nuts are the fats of greatest importance as foods. Fats, like the
carbohydrates, are composed of carbon, hydrogen, and oxygen. They are
rather complex chemical compounds, though not so complex as proteids.
Since neither fats nor carbohydrates contain nitrogen, they are
frequently classed together as non-nitrogenous foods.

Purpose Served by Carbohydrates, Fats,
and Albuminoids.—These classes of nutrients all serve the common
purpose of supplying energy. By uniting with oxygen at the cells, they
supply heat and the other forms of bodily force. This is perhaps their
only purpose.50 Proteids also serve this purpose, but they are not so
well adapted to supplying energy as are the carbohydrates and the fats.
In the first place they do not completely oxidize and therefore do not
supply so much energy; and, in the second place, they form waste
products that are removed with difficulty from the body.

Mineral Salts and their
Uses.—Mineral salts are found in small quantities in all of the
more common food materials, and, as a rule, find their way into the body
unnoticed. They supply the elements which are found in the body in small
quantities and serve a variety of [pg 122]purposes.51
Calcium phosphate and
calcium carbonate are important constituents of the bones and teeth; and
the salts containing iron renew the hemoglobin of the blood. Others
perform important functions in the vital processes. The mineral compound
of greatest importance perhaps is sodium chloride, or common salt.52
This is a natural constituent of most of our foods, and is also added to
food in its preparation for the table. When it is withheld from animals
for a considerable length of time, they suffer intensely and finally
die. It is necessary in the blood and lymph to keep their constituents
in solution, and is thought to play an important rôle in the chemical
changes of the cells. It is constantly leaving the body as a waste
product and must be constantly supplied in small quantities in the
foods.

[pg 123]Importance of Water.—Water finds
its way into the body as a pure liquid, as a part of such mixtures as
coffee, chocolate, and milk, and as a constituent of all our solid
foods. (See table of foods, page 126.) It is also formed in the body by
the oxidation of hydrogen. It passes through the body unchanged, and is
constantly being removed by all the organs of excretion. Though water
does not liberate energy in the body nor build up the tissues in the
sense that other foods do, it is as necessary to the maintenance of life
as oxygen or proteids. It occurs in all the tissues, and forms about 70
per cent of the entire weight of the body. Its presence is necessary for
the interchange of materials at the cells and for keeping the tissues
soft and pliable. As it enters the body, it carries digested food
substances with it, and as it leaves it is loaded with wastes. Its chief
physiological work, which is that of a transporter of material, depends upon its ability to
dissolve substances and to flow readily from place to place.

Relative Quantity of Nutrients
Needed.—Proteids, carbohydrates, and fats are the nutrients that
supply most of the body’s nourishment. The most hygienic diet is the one
which supplies the proteids in sufficient quantity to rebuild the
tissues and the carbohydrates and fats in the right amounts to supply
the body with energy. Much experimenting has been done with a view to
determining these proportions, but the results so far are not entirely
satisfactory. According to some of the older estimates, a person of
average size requires for his daily use five ounces of proteid, two and
one half ounces of fat, and fifteen ounces of carbohydrate. Recent
investigations of this problem seem to show that the body is as well, if
not better, nourished by a much smaller amount of proteid—not more than
two and one half ounces (60 grams) daily.53

[pg 124] While there is probably no
necessity for the healthy individual’s taking his proteid, fat, and
carbohydrate in exact proportions (if
the proportions best suited to his body were known), the fact needs to
be emphasized that proteids, although absolutely necessary, should form
but a small part (not over one fifth) of the daily bill of fare. In
recognition of this fact is involved a principle of health and also one
of economy. The proteids, especially those in meats, are the most
expensive of the nutrients, whereas the carbohydrates, which should form
the greater bulk of one’s food, are the least expensive.

Effects of a One-sided Diet.—The
plan of the body is such as to require a mixed diet, and all of the great classes of nutrients are
necessary. If one could subsist on any single class, it would be
proteids, for proteids are able both to rebuild tissue and to supply
energy. But if proteids are eaten much in excess of the body’s need for
rebuilding the tissues, and this excess is oxidized for supplying
energy, a strain is thrown upon the organs of excretion, because of the
increase in the wastes. Not only is there danger of overworking certain
of these organs (the liver and kidneys), but the wastes may linger too
long in the body, causing disorder and laying the foundation for
disease. On the other hand, if an insufficient amount of proteid is
taken, the tissues are improperly nourished, and one is unable to exert
his usual strength. What is true of the proteids is true, though in a
different way, of the other great classes of foods. A diet which is
lacking in proteid, carbohydrate, or fat, or which has any one of them
in excess, is not adapted to the requirements of the body.

Composition of the Food Materials.—One
who intelligently provides the daily bill of fare must have some
knowledge of the nature and quantity of the nutrients[pg 125] present in the different
materials used as food. This information is supplied by the chemist, who
has made extensive analyses for this purpose. Results of such analyses
are shown in Table 1 (page 126), which gives the percentage of proteids,
fats, carbohydrates, water, and mineral salts in the edible portions of
the more common of our foods.

Fig. 61—Relative proportions of different nutrients in well-known foods.

Food Supply to the Table.—The main
problem in supplying the daily bill of fare is that of securing through
the different food materials the requisite amounts of proteids,
carbohydrates, and fats. In this matter a table showing the composition
of foods can be used to great advantage. Consulting the table on page
126, it is seen that large per cents of proteids are supplied by lean
meat, eggs, cheese, beans, peas, peanuts, and oatmeal, while fat is in
excess in fat meat, butter, and nuts (Fig. 61). Carbohydrates are
supplied in abundance by potatoes, rice, corn, sugar, and molasses. The
different cereals also contain a large percentage of carbohydrates in
the form of starch.

[pg 126]

[pg 128]Variety in the selection of foods for the table is an
essential feature, but this should not increase either the work or the
expense of supplying the meals. Each single meal can, and should, be
simple in itself and, at the same time, differ sufficiently from the
meal preceding and the one following to give the necessary variety in
the course of the day. The bill of fare should, of course, include
fruits (for their tonic effects) and very small amounts perhaps of
substances which stimulate the appetite, such as pepper, mustard, etc.,
known as condiments.

Purity of Food.—The fact that many
of the food substances are perishable makes it possible for them to be
eaten in a slightly decayed condition. Such substances are decidedly
unwholesome (some containing poisons) and should be promptly rejected.
Not only do fresh meats, fruits, and vegetables need careful inspection,
but canned and preserved goods as well. If canned foods are imperfectly
sealed or if not thoroughly cooked in the canning process, they decay
and the acids which they generate act on the metals lining the cans,
forming poisonous compounds. The contents of “tin” cans should for this
reason be transferred to other vessels as soon as opened.

Foods are also rendered impure or weakened through adulteration, the
watering of milk being a familiar example. The manufacture of jellies,
preserves, sirups, and various kinds of pickles and condiments has
perhaps afforded the largest field for adulterations, although it is
possible to adulterate nearly all of the leading articles of food. A
long step in the prevention of food and drug adulteration was taken in
this country by the passage of the Pure
Food Law. By forcing manufacturers of foods and medicines to state
on printed labels the composition of their products, this law has made
it possible for the consumer to know what he is purchasing and putting
into his body.

Alcohol not a Food.—Many people in
this and other countries drink in different beverages, such as whisky,
beer, wine, etc., a varying amount of alcohol. This substance has a
temporary stimulating or exciting effect, and the claim has been made
that it serves as a food. Recently[pg 129] it has been shown that alcohol
when introduced into the body in small quantities and in a greatly
diluted form, is nearly all oxidized, yielding energy as does fat or
sugar. If no harmful effects attended the use of alcohol, it might on
this account be classed as a food. But alcohol is known to be harmful to
the body. When used in large quantities, it injures nearly all of the
tissues, and when taken habitually, even in small doses, it leads to the
formation of the alcohol habit which is now recognized and treated as a
disease. This and other facts show that alcohol is not adapted to the
body plan of taking on and using new material (Chapter XI), and no
substance lacking in this respect can properly be classed as a food.56
Instead of classing alcohol as a food, it should be placed in that long
list of substances which are introduced into the body for special
purposes and which are known by the general name of

Drugs.—Drugs act strongly upon the
body and tend to bring about unusual and unnatural results. Their use
should in no way be confused with that of foods. If taken in health,
they tend to disturb the physiological balance of the body by unduly
increasing or diminishing the action of the different organs. In disease
where this balance is already disturbed, they may be administered for
their counteractive effects, but always under the advice and direction
of a physician. Knowing the nature of the disturbance which the drug
produces, the physician can administer it to advantage, should the body
be out of physiological[pg 130] balance, or diseased. Not only are drugs of no value in
health, but their use is liable to do much harm.

NATURE OF DIGESTION

Before the nutrients can be oxidized at the cells, or built into the
protoplasm, they undergo a number of changes. These are necessary for
their entrance into the body, for their distribution by the blood and
the lymph, and for the purposes which they finally serve. The first of
these changes is preparatory to the entrance of the nutrients and is
known as digestion. The organs which
bring about this change, called digestive organs, have a special
construction which adapts them to their work. It will assist materially
in understanding these organs if we first learn something of the nature
of the work which they have to perform.

How the Nutrients get into the
Body.—The nature of digestion is determined by the conditions
affecting the entrance of nutrients into the body. Food in the stomach
and air in the lungs, although surrounded by the body, are still outside
of what is called the body proper. To
gain entrance into the body proper, a substance must pass through the
body wall. This consists of the skin on the outside and of the mucous
linings of the air passages and other tubes and cavities which are
connected with the external surface.

To get from the digestive organs into the blood, the nutrients must
pass through the mucous membrane lining these organs and also the walls
of blood or lymph vessels. Only liquid
materials can make this passage. It is necessary, therefore, to
reduce to the liquid state all nutrients not already in that condition.
This reduction to the liquid state
constitutes the digestive process.

[pg 131]How Substances are Liquefied.—While
the reduction of solids to the liquid state is accomplished in some
instances by heating them until they melt, they are more frequently
reduced to this state by subjecting them to the action of certain
liquids, called solvents. Through the
action of the solvent the minute particles of the solid separate from
each other and disappear from view. (Shown in dropping salt in water.)
At the same time they mix with the solvent, forming a solution, from which they separate only
with great difficulty. For this reason solids in solution can diffuse
through porous partitions along with the solvents in which they are
dissolved (page 73).

By digestion the nutrients are reduced to the form of a solution. The process is, simply speaking, one of dissolving. The liquid employed as
the digestive solvent is water. The
different nutrients dissolve in water, mixing with it to form a solution
which is then passed into the body proper.

Digestion not a Simple
Process.—Digestion is by no means a simple process, such, for
instance, as the dissolving of salt or sugar in water. These, being
soluble in water, dissolve at once on being mixed with a sufficient
amount of this liquid. The majority of the nutrients, however, are
insoluble in water and are unaffected by it when acting alone. Fats,
starch, and most of the proteids do not dissolve in water. Before these
can be dissolved they have to be changed chemically and converted into
substances that are soluble in water.
This complicates the process and prevents
the use of water alone as the digestive solvent.

A Similar Case.—If a piece of
limestone be placed in water, it does not dissolve, because it is
insoluble in water. If hydrochloric acid is now added to the water, the
limestone[pg 132] is soon dissolved (Fig.
62). (See Practical Work.) It seems at first thought that the acid
dissolves the limestone, but this is not the case. The acid produces a
chemical change in the limestone (calcium carbonate) and converts it
into a compound (calcium chloride) that is soluble in water. As fast as
this is formed it is dissolved by the water, which is the real solvent
in the case. The acid simply plays the part of a chemical converter.

Fig. 62—The dissolving of limestone in water
containing acid, suggesting the double action in the digestion of most
foods.

The Digestive Fluids.—Several
fluids—saliva, gastric juice, pancreatic juice, bile, and intestinal
juice—are employed in the digestion of the food. The composition of
these fluids is in keeping with the nature of the digestive process.
While all of them have water for their most abundant constituent, there
are dissolved in the water small amounts of active chemical agents. It
is the work of these agents to convert the insoluble nutrients into
substances that are soluble in water. The digestive fluids are thus able
to act in a double manner on the
nutrients—to change them chemically and to dissolve them. The chemical
agents which bring about the changes in the nutrients are called enzymes, or digestive ferments.

Foods Classed with Reference to
Digestive Changes.—With reference to the changes which they undergo
during digestion, foods may be divided into three classes as
follows:

1. Substances already in the liquid state and requiring no digestive
action. Water and solutions of simple foods in water belong to this
class. Milk and liquid fats, or oils, do not belong to this class.

2. Solid foods soluble in water. This class includes[pg 133] common salt and sugar. These
require no digestive action other than dissolving in water.

3. Foods that are insoluble in water. These have first to be changed
into soluble substances, after which they are dissolved.

Summary.—Materials called foods
are introduced into the body for rebuilding the tissues, supplying
energy, and aiding in its general work. Only a few classes of
substances, viz., proteids, carbohydrates, fats, water, and some mineral
compounds have all the qualities of foods and are suitable for
introduction into the body. Substances known as drugs, which may be used
as medicines in disease, should be avoided in health. Before foods can
be passed into the body proper, they must be converted into the liquid
form, or dissolved. In this process, known as digestion, water is the
solvent; and certain chemical agents, called enzymes, convert the
insoluble nutrients into substances that are soluble in water.

Exercises.—1. How does oxidation at
the cells make necessary the introduction of new materials into the
body?

2. What different purposes are served by the foods?

3. What is a nutrient? Name the important classes.

4. What are food materials? From what sources are they obtained?

5. Name the different kinds of proteids; the different kinds of
carbohydrates. Why are proteids called nitrogenous foods and fats and
carbohydrates non-nitrogenous foods?

6. Show why life cannot be carried on without proteids; without
water.

7. What per cents of proteid, fat, and carbohydrate are found in
wheat flour, oatmeal, rice, butter, potatoes, round beef, eggs, and
peanuts?

8. State the objection to a meal consisting of beef, eggs, beans,
bread, and butter; to one consisting of potatoes, rice, bread, and
butter. Which is the more objectionable of these meals and why?

9. State the general plan of digestion.

[pg 134]10. Show that digestion is not a
simple process like that of dissolving salt in water.

CHAPTER X – ORGANS AND PROCESSES OF DIGESTION

The organs of digestion are adapted to the work of dissolving the
foods by both their structure and arrangement. Most of them consist
either of tubes or cavities and these are so connected, one with the
other, as to form a continuous passageway entirely through the body.
This passageway is known as

The Alimentary Canal. —The
alimentary canal has a length of about thirty feet and, while it begins
at the mouth, all but about eighteen inches of it is found in the
abdominal cavity. On account of its length it lies for the most part in
coils, the two largest ones being known as the small intestine and the
large intestine. Connected with the alimentary canal are the glands that
supply the liquids for acting on the food. The divisions of the canal
and most of the glands that empty liquids into it are shown in Fig. 63
and named in the table below:

[pg 139]Coats of the Alimentary Canal.—The walls of the alimentary
canal, except at the mouth, are distinct from the surrounding tissues
and consist in most places of at least three layers, or coats, as
follows:

Fig. 63—Diagram of the
digestive system. 1. Mouth. 2. Soft palate. 3. Pharynx. 4. Parotid
gland. 5. Sublingual gland. 6. Submaxillary gland. 7. Esophagus. 8.
Stomach. 9. Pancreas. 10. Vermiform appendix. 11. Cæcum. 12. Ascending
colon. 13. Transverse colon. 14. Descending colon. 15. Sigmoid flexure.
16. Rectum. 17. Ileo-cæcal valve. 18. Duct from liver and pancreas. 19.
Liver.

Diagram does not show comparative length of the small intestine.

1. An inner coat, or lining, known
as the mucous membrane. This membrane is not confined to the alimentary
canal, but lines, as we have seen, the different air passages. It
covers, in fact, all those internal surfaces of the body that connect
with the external surface. It derives its name from the substance which
it secretes, called mucus. In
structure it resembles the skin, being continuous with the skin where
cavities open to the surface. It is made up of two layers—a thick
underlayer which contains blood vessels, nerves, and glands, and a thin
surface layer, called the epithelium.
The epithelium, like the cuticle, is without blood vessels, nerves, or
glands.

2. A middle coat, which is
muscular and which forms a continuous layer throughout the canal, except
at the mouth. (Here its place is taken by the strong muscles of
mastication which are separate and distinct from each other.) As a[pg 140] rule the muscles of this coat are
involuntary. They surround the canal as thin sheets and at most places
form two distinct layers. In the inner layer the fibers encircle the
canal, but in the outer layer they run longitudinally, or lengthwise,
along the canal.57

3. An outer or serous coat, which is limited to those
portions of the canal that occupy the abdominal cavity. This coat is not
found above the diaphragm. It is a part of the lining membrane of the
cavity of the abdomen, called

Fig. 64—Diagram of the
peritoneum. 1. Transverse colon. 2. Duodenum. 3. Small intestine.
4. Pancreas.

The Peritoneum.—The peritoneum is
to the abdominal cavity what the pleura is to the thoracic cavity. It
forms the outer covering for the alimentary canal and other abdominal
organs and supplies the inner lining of the cavity itself. It is also
the means of holding these organs in place, some of them being suspended
by it from the abdominal walls (Fig. 64). By the secretion of a small
amount of liquid, it prevents friction of the parts upon one
another.

Digestive Glands.—The glands which
provide the different fluids for acting on the foods derive their
constituents from the blood. They are situated either in the mucous
membrane or at convenient places outside of the[pg 141] canal and pass their liquids into
it by means of small tubes, called ducts. In the canal the food and the
digestive fluids come in direct contact—a condition which the dissolving
processes require. Each kind of fluid is secreted by a special kind of
gland and is emptied into the canal at the place where it is needed.

The Digestive Processes.—Digestion
is accomplished by acting upon the food in different ways, as it is
passed along the canal, with the final result of reducing it to the form
of a solution. Several distinct processes are necessary and they occur
in such an order that those preceding are preparatory to those that
follow. These processes are known as mastication, insalivation, deglutition, stomach digestion,
and intestinal digestion. As the
different materials become liquefied they are transferred to the blood,
and substances not reduced to the liquid state are passed on through the
canal as waste. The first two of the digestive processes occur in

The Mouth.—This is an oval-shaped
cavity situated at the very beginning of the canal. It is surrounded by
the lips in front, by the cheeks on the sides, by the hard palate above
and the soft palate behind, and by the tissues of the lower jaw below.
The mucous membrane lining the mouth is, soft and smooth, being covered
with flat epithelial cells. The external opening of the mouth is guarded
by the lips, and the soft palate forms a movable partition between the mouth and the pharynx. In a
condition of repose the mouth space is practically filled by the teeth
and the tongue, but the cavity may be enlarged and room provided for
food by depressing the lower jaw.

The mouth by its construction is well adapted to carrying on the
processes of mastication and insalivation. By the first process the
solid food is reduced, by the cutting[pg 142] and grinding action of the teeth,
to a finely divided condition. By the second, the saliva becomes mixed
with the food and is made to act upon it.

Fig. 65—The teeth.
A. Section of a single molar. 1.
Pulp. 2. Dentine. 3. Enamel. 4. Crown. 5. Neck. 6. Root. B. Teeth in position in lower jaw. 1.
Incisors. 2. Canine. 3. Biscuspids. 4. Molars. C. Upper and lower teeth on one side. 1. Incisors. 2.
Canines. 3. Biscuspids. 4. Molars. 5. Wisdom. D. Upper and lower incisor, to show gliding contact.

Accessory Organs of the Mouth.—The
work of mastication and insalivation is accomplished through organs
situated in and around the mouth cavity. These comprise:

1. The Teeth.—The teeth are set in
the upper and lower jaws, one row directly over the other, with their
hardened surfaces facing. In reducing the food, the teeth of the lower
jaw move against those of the upper, while the food is held by the
tongue and cheeks between the grinding surfaces. The front teeth are
thin and chisel-shaped. They do not meet so squarely as do the back
ones, but their edges glide over each other, like the blades of
scissors—a condition that adapts them to cutting off and separating the
food (D, Fig. 65). The back teeth are
broad and irregular, having surfaces that are adapted to crushing and
grinding.

Each tooth is composed mainly of a bone-like substance, called dentine, which surrounds a central space,
containing blood vessels and[pg 143] nerves,
known as the pulp cavity. It is set
in a depression in the jaw where it is held firmly in place by a bony
substance, known as cement. The part
of the tooth exposed above the gum is the crown, the part surrounded by the gum is the neck, and the part which penetrates into
the jaw is the root (A, Fig. 65). A hard, protective material,
called enamel, covers the exposed
surface of the tooth.

The teeth which first appear are known as the temporary, or milk, teeth and are twenty in number, ten in
each jaw. They usually begin to appear about the sixth month, and they
disappear from the mouth at intervals from the sixth to the thirteenth
year. As they leave, teeth of the second, or permanent, set take their place. This set has thirty-two
teeth of four different kinds arranged in the two jaws as follows:

In front, above and below, are four chisel-shaped teeth, known as the
incisors. Next to these on either
side is a tooth longer and thicker than the incisors, called the canine. Back of these are two short,
rounded and double pointed teeth, the bicuspids, and back of the bicuspids are three heavy teeth
with irregular grinding surfaces, called the molars (B and C, Fig. 65). Since the molar farthest
back in each jaw is usually not cut until maturity, it is called a wisdom tooth. The molars are known as the
superadded permanent teeth because they do not take the place of milk
teeth, but form farther back as the jaw grows in length.

Fig. 66—Diagram
showing directions of muscular fibers in tongue.

2. The Tongue.—The tongue is a
muscular organ whose fibers extend through it in several directions
(Fig. 66). Its structure adapts it to a variety of movements. During
mastication the tongue transfers the food from one part of the mouth to
another, and, with the aid of the cheeks, holds the food between the
rows of teeth. (By an outward pressure from the tongue and an inward
pressure from the cheek the food is kept between the grinding surfaces.)
The tongue has functions in addition to these and is a most useful
organ.

[pg 144]3. The Muscles of Mastication.—These are attached to the lower
jaw and bring about its different movements. The masseter muscles, which are the heavy muscles in the
cheeks, and the temporal muscles,
located in the region of the temples, raise the lower jaw and supply the
force for grinding the food. Small muscles situated below the chin
depress the jaw and open the mouth.

Fig. 67—Salivary
glands and the ducts connecting them with the mouth.

4. The Salivary Glands.—These
glands are situated in the tissues surrounding the mouth, and
communicate with it by means of ducts (Fig. 67). They secrete the
saliva. The salivary glands are six in number and are arranged in three
pairs. The largest, called the parotid glands, lie, one on either side, in front of and
below the ears. A duct from each gland passes forward along the cheek
until it opens in the interior of the mouth, opposite the second molar
tooth in the upper jaw. Next in size to the parotids are the submaxillary glands. These are located,
one on either side, just below and in front of the triangular bend in
the lower jaw. The smallest of the salivary glands are the sublingual. They are situated in the
floor of the mouth, on either side, at the front and base of the tongue.
Ducts from the submaxillary and sublingual glands open into the mouth
below the tip of the tongue.

The Saliva and its Uses.—The saliva
is a transparent and somewhat slimy liquid which is slightly alkaline.
It[pg 145] consists chiefly of water (about
99 per cent), but in this are dissolved certain salts and an active
chemical agent, or enzyme, called ptyalin, which acts on the starch. The ptyalin changes
starch into a form of sugar (maltose), while the water in the saliva
dissolves the soluble portions of the food. In addition to this the
saliva moistens and lubricates the food which it does not dissolve, and
prepares it in this way for its passage to the stomach. The last is
considered the most important use of the saliva, and dry substances,
such as crackers, which require a considerable amount of this liquid,
cannot be eaten rapidly without choking. Slow mastication favors the
secretion and action of the saliva.

Deglutition.—Deglutition, or
swallowing, is the process by which food is transferred from the mouth
to the stomach. Though this is not, strictly speaking, a digestive
process, it is, nevertheless, necessary for the further digestion of the
food. Mastication and insalivation, which are largely mechanical,
prepare the food for certain chemical processes by which it is
dissolved. The first of these occurs in the stomach and to this organ
the food is transferred from the mouth. The chief organs concerned in
deglutition are the tongue, the pharynx, and the esophagus.

The Pharynx is a round and somewhat
cone-shaped cavity, about four and one half inches in length, which lies
just back of the nostrils, mouth, and larynx. It is remarkable for its
openings, seven in number, by means of which it communicates with other
cavities and tubes of the body. One of these openings is into the mouth,
one into the esophagus, one into the larynx, and one into each of the
nostrils, while two small tubes (the eustachian) pass from the upper
part of the pharynx to the middle ears.

The pharynx is the part of the food canal that is crossed[pg 146] by the passageway for the air. To
keep the food from passing out of its natural channel, the openings into
the air passages have to be carefully guarded. This is accomplished
through the soft palate and epiglottis, which are operated somewhat as
valves. The muscular coat of the pharynx is made up of a series of
overlapping muscles which, by their contractions, draw the sides
together and diminish the cavity. The mucous membrane lining the pharynx
is smooth, like that of the mouth, being covered with a layer of flat
epithelial cells.

The Esophagus, or gullet, is a tube
eight or nine inches long, connecting the pharynx with the stomach. It
lies for the most part in the thoracic cavity and consists chiefly of a
thick mucous lining surrounded by a heavy coat of muscle. The muscular
coat is composed of two layers—an inner layer whose fibers encircle the
tube and an outer layer whose fibers run lengthwise.

Steps in Deglutition.—The process
of deglutition varies with the kind of food. With bulky food it consists
of three steps, or stages, as follows: 1. By the contraction of the
muscles of the cheeks, the food ball, or bolus, is pressed into the
center of the mouth and upon the upper surface of the tongue. Then the
tongue, by an upward and backward movement, pushes the food under the
soft palate and into the pharynx.

2. As the food passes from the mouth, the pharynx is drawn up to
receive it. At the same time the soft palate is pushed upward and
backward, closing the opening into the upper pharynx, while the
epiglottis is made to close the opening into the larynx. By this means
all communication between the food canal and the air passages is
temporarily closed. The upper muscles of the pharynx now contract upon
the food, forcing it downward and into the esophagus.

3. In the esophagus the food is forced along by the successive
contractions of muscles, starting at the upper end of the tube, until
the stomach is reached.

Swallowing is doubtless aided to some extent by the force of
gravity. [pg 147]That it is independent of this
force, however, is shown by the fact that one may swallow with the
esophagus in a horizontal position, as in lying down.

Fig. 68—Gastric
Glands. A. Single gland showing
the two kinds of secreting cells and the duct where the gland opens on
to the surface. B. Inner surface of
stomach magnified. The small pits are the openings from the glands.

The Stomach.—The stomach is the
largest dilatation of the alimentary canal. It is situated in the
abdominal cavity, immediately below the diaphragm, with the larger
portion toward the left side. Its connection with the esophagus is known
as the cardiac orifice and its
opening into the small intestine is called the pyloric orifice. It varies greatly in size in different
individuals, being on the average from ten to twelve inches at its
greatest length, from four to five inches at its greatest width, and
holding from three to five pints. It has the coats common to the canal,
but these are modified somewhat to adapt them to its work.

The mucous membrane of the stomach
is thick and highly developed. It contains great numbers of minute
tube-shaped bodies, known as the gastric
glands (Fig. 68). These are of two general kinds and secrete large
quantities of a liquid called the gastric juice. When the stomach is
empty, the mucous membrane is thrown into folds which run lengthwise
over the inner surface. These disappear, however, when the walls of the
stomach are distended with food.

[pg 148]The muscular coat consists of three separate layers which are named,
from the direction of the fibers, the circular layer, the longitudinal
layer, and the oblique layer (Fig. 69).
The circular layer becomes quite thick at the pyloric orifice,
forming a distinct band which serves as a valve.

Fig. 69—Muscles of the
stomach (from Morris’ Human
Anatomy). The layer of Longitudinal fibers removed.

The outer coat of the stomach, called the serous coat, is a continuation of the peritoneum, the
membrane lining the abdominal cavity.

Stomach Digestion.—In the stomach
begins the definite work of dissolving those foods which are insoluble
in water. This, as already stated, is a double process. There is first a
chemical action in which the insoluble are changed into soluble
substances, and this is followed immediately by the dissolving action of
water. The chief substances digested in the stomach are the proteids.
These, in dissolving, are changed into two soluble substances, known
as[pg 149] peptones and proteoses.
The digestion of the proteids is, of course, due to the

Gastric Juice.—The gastric juice is
a thin, colorless liquid composed of about 99 per cent of water and
about 1 per cent of other substances. The latter are dissolved in the
water and include, besides several salts, three active chemical
agents—hydrochloric acid, pepsin, and rennin. Pepsin is the enzyme which acts upon proteids, but it is
able to act only in an acid medium—a condition which is supplied by the
hydrochloric acid. Mixed with the
hydrochloric acid it converts the proteids into peptones and
proteoses.

Other Effects of the Gastric
Juice.—In addition to digesting proteids, the gastric juice brings
about several minor effects, as follows:

1. It checks, after a time, the digestion of the starch which was
begun in the mouth by the saliva.58 This is due to the presence of the
hydrochloric acid, the ptyalin being unable to act in an acid
medium.

2. While there is no appreciable action on the fat itself, the
proteid layers that inclose the fat particles are dissolved away (Fig.
79), and the fat is set free. By this means the fat is broken up and
prepared for a special digestive action in the small intestine.

3. Dissolved albumin, like that in milk, is curded, or coagulated, in
the stomach. This action is due to the rennin. The curded mass is then acted upon by the pepsin
and hydrochloric acid in the same manner as the other proteids.

[pg 150]4. The hydrochloric acid acts on certain of the insoluble mineral
salts found in the foods and reduces them to a soluble condition.

5. It is also the opinion of certain physiologists that cane sugar
and maltose (double sugars) are converted by the hydrochloric acid into
dextrose and levulose (single sugars).

After a variable length of time, the contents of the stomach is
reduced to a rather uniform and pulpy mass which is called chyme. Portions of this are now passed at
intervals into the small intestine.

Muscular Action of the Stomach.—The
muscles in the walls of the stomach have for one of their functions the
mixing of the food with the gastric juice. By alternately contracting and relaxing, the different layers
of muscle keep the form of the stomach changing—a result which agitates
and mixes its contents. This action varies in different parts of the
organ, being slight or entirely absent at the cardiac end, but quite
marked at the pyloric end.

Another purpose of the muscular coat is to empty the stomach into the
small intestine. During the greater part of the digestive period the
muscular band at the pyloric orifice is contracted. At intervals,
however, this band relaxes, permitting a part of the contents of the
stomach to be forced into the small intestine. After the discharge the
pyloric muscle again contracts, and so remains until the time arrives
for another discharge.

In addition to emptying the stomach into the small intestine, these
muscles also aid in emptying the organ upward and through the esophagus
and mouth, should occasion require. Vomiting in case of poisoning, or if
the food for some reason fails to digest, is a necessary though
unpleasant operation. It is accomplished by the[pg 151] contraction of all the muscles of
the stomach, together with the contraction of the walls of the abdomen.
During these contractions the pyloric valve is closed, and the muscles
of the esophagus and pharynx are in a relaxed condition.59

Fig. 70—Passage from
stomach into small intestine. Illustration also shows arrangement
of mucous membrane in the two organs. D. Bile duct.

The Small Intestine.—This division
of the alimentary canal consists of a coiled tube, about twenty-two feet
in length, which occupies the central, lower portion of the abdominal
cavity (Fig. 71). At its upper extremity it connects with the pyloric
end of the stomach (Fig. 70), and at its lower end it joins the large
intestine. It averages a little over an inch in diameter, and gradually
diminishes in size from the stomach to the large intestine. The first
eight or ten inches form a short curve, known as the duodenum. The upper two fifths of the
remainder is called the jejunum, and
the lower three fifths is known as the ileum. The ileum joins that part of the large intestine
known as the cæcum, and at their place of union is a marked constriction
which prevents material from passing from the large into the small
intestine (Fig. 73). This is known as the ileo-cæcal valve.

The mucous membrane of the small
intestine is richly supplied with blood vessels and contains glands that
secrete[pg 152] a digestive fluid known as the
intestinal juice. The membrane is
thrown into many transverse, or circular, folds which increase its
surface and also prevent materials from passing too rapidly through the
intestine. One important respect in which the small intestine differs
from all other portions of the food canal is that its surface is covered
with great numbers of minute elevations known as the villi. The purpose
of these is to aid in the absorption of the nutrients as they become
dissolved (Chapter XI).

The muscular coat of the small
intestine is made up of two distinct layers—the inner layer consisting
of circular fibers and the outer of longitudinal fibers. These muscles
keep the food materials mixed with the juices of the small intestine,
but their main purpose is to force the materials undergoing digestion
through this long and much-coiled tube.

The outer, or serous, coat of the
small intestine, like that of the stomach, is an extension from the
general lining of the abdominal cavity, or peritoneum. In fact, the
intestine lies in a fold of the peritoneum, somewhat as an arm in a
sling, while the peritoneum, by connecting with the back wall of the
abdominal cavity, holds this great coil of digestive tubing in place
(Fig. 64). The portion of the peritoneum which attaches the intestine to
the wall of the abdomen is called the mesentery.

Most of the liquid acting on the food in the small intestine is
supplied by two large glands, the liver and the pancreas, that connect
with it by ducts.

Fig. 71—Abdominal cavity with organs of digestion
in position.

The Liver is situated immediately
below the diaphragm, on the right side (Figs. 71 and 72), and is the
largest gland in the body. It weighs about four pounds and is separated
into two main divisions, or lobes. It is complex in structure and
differs from the other glands in several particulars. It receives blood
from two distinct sources—the portal vein[pg 154] and the hepatic artery. The portal vein collects the blood from
the stomach, intestines, and spleen, and passes it to the liver. This
blood is loaded with food materials, but contains little or no oxygen.
The hepatic artery, which branches
from the aorta, carries to the liver blood rich in oxygen. In the liver
the portal vein and the hepatic artery divide and subdivide, and finally
empty their blood into a single system of capillaries surrounding the
liver cells. These capillaries in turn empty into a single system of
veins which, uniting to form the hepatic
veins (two or three in number), pass the blood into the inferior
vena cava (Fig. 72).

Fig. 72—Relations of the
liver. Diagram showing the connection of the liver with the large
blood vessels and the food canal.

The liver secretes daily from one to two pounds of a liquid called
bile. A reservoir for the bile is
provided by a small, membranous sack, called the gall bladder, located on the underside of the liver. The
bile passes from the gall bladder, and from the right and left lobes of
the liver, by three separate ducts. These unite to form a common tube
which, uniting with the duct from the pancreas, empties into the
duodenum. Though usually described as a digestive gland, the liver has
other functions of equal or greater importance (Chapter XIII).

[pg 155]The
Bile is a golden yellow liquid, having a slightly alkaline reaction
and a very bitter taste. It consists, on the average, of about 97 per
cent of water and 3 per cent of solids.60 The solids include bile
pigments, bile salts, a substance called cholesterine, and mineral
salts. The pigments (coloring matter) of the bile are derived from the
hemoglobin of broken-down red corpuscles (page 27).

Much about the composition of the bile is not understood. It is
known, however, to be necessary to digestion, its chief use being to aid
in the digestion and absorption of fats. It is claimed also that the
bile aids the digestive processes in some general ways—counteracting the
acid of the gastric juice, preventing the decomposition of food in the
intestines, and stimulating muscular action in the intestinal walls. No
enzymes have been discovered in the bile.

The Pancreas is a tapering and
somewhat wedge-shaped gland, and is so situated that its larger
extremity, or head, is encircled by the duodenum. From here the more
slender portion extends across the abdominal cavity nearly parallel to
and behind the lower part of the stomach. It has a length of six or
eight inches and weighs from two to three and one half ounces. Its
secretion, the pancreatic juice, is emptied into the duodenum by a duct
which, as a rule, unites with the duct from the liver.

The Pancreatic Juice is a colorless
and rather viscid liquid, having an alkaline reaction. It consists of
about 97.6 per cent of water and 2.4 per cent of solids. The solids
include mineral salts (the chief of which is sodium carbonate) and four
different chemical agents, or enzymes,—trypsin, amylopsin, steapsin, and
a milk-curding enzyme. These active constituents make of the pancreatic
juice the[pg 156] most important of the digestive
fluids. It acts with vigor on all of the nutrients insoluble in water,
producing the following changes:

1. It converts the starch into maltose, completing the work begun by
the saliva. This action is due to the amylopsin,61 which is similar to ptyalin but is more
vigorous.

2. It changes proteids into peptones and proteoses, completing the
work begun by the gastric juice. This is accomplished by the trypsin, which is similar to, but more
active than, the pepsin.

3. It digests fat. In this work the active agent is the steapsin.

The necessity of a milk-curding enzyme, somewhat similar to the
rennin of the gastric juice, is not understood.

Digestion of Fat.—Several theories
have been proposed at different times regarding the digestion and
absorption of fat. Among these, what is known as the “solution theory”
seems to have the greatest amount of evidence in its favor. According to
this theory, the fat, under the influence of the steapsin, absorbs water
and splits into two substances, recognized as glycerine and fatty acid.
This finishes the process so far as the glycerine is concerned, as this
is soluble in water; but the fatty acid, which (from certain fats) is
insoluble in water,62 requires further treatment. The fatty acid is now
supposed to be acted on in one, or both, of the following ways: 1. To be
dissolved as fatty acid by the action of the bile (since bile is
capable[pg 157] of dissolving it under certain
conditions). 2. To be converted by the sodium carbonate into a form of
soap which is soluble in water.

The emulsification of fat is known to occur in the small intestine.
By this process the fat is separated into minute particles which are
suspended in water, but not changed chemically, the mixture being known
as an emulsion. While this is
believed by some to be an actual process of digestion, the advocates of
the solution theory claim that it is a process accompanying and aiding
the conversion of fat into fatty acid and glycerine.63

The Intestinal Juice is a clear
liquid with an alkaline reaction, containing water, mineral salts, and
certain proteid substances that may act as enzymes. It assists in
bringing about an alkaline condition in the small intestine and aids in
the reduction of cane sugar and maltose to the simple sugars, dextrose
and levulose. Since it is difficult to obtain this liquid in sufficient
quantities for experimenting, its uses have not been fully determined.
Recent investigators, however, assign to it an important place in the
work of digestion.

Work of the Small Intestine.—The
small intestine is the most important division of the alimentary canal.
It serves as a receptacle for holding the food while it is being acted
upon; it secretes the intestinal juice and mixes the food with the
digestive fluids; it propels the food toward the large intestine; and,
in addition to all this, serves as an organ of absorption.

Digestion is practically finished in the small intestine, and a large
portion of the reduced food is here absorbed. There is always present,
however, a variable amount of material that is not digested. This,
together with a considerable volume of liquid, is passed into

[pg 158]The
Large Intestine.—The large intestine is a tube from five to six
feet in length and averaging about one and one half inches in diameter.
It begins at the lower right side of the abdominal cavity, forms a coil
which almost completely surrounds the coil of small intestine, and
finally terminates at the surface of the body (Figs. 2, 71 and 73). It
has three divisions, known as the cæcum, the colon, and the rectum.

Fig. 73—Passage from
small into large intestine. At the ileo-cæcal valve is the narrowest
constriction of the food canal.

The cæcum is the pouch-like
dilatation of the large intestine which receives the lower end of the
small intestine. It measures about two and one half inches in diameter
and has extending from one side a short, slender, and blind tube, called
the vermiform appendix. This
structure serves no purpose in digestion, but appears to be the rudiment
of an organ which may have served a purpose at some remote period in the
history of the human race. The cæcum gradually blends into the second
division of the large intestine, called the colon.

The colon consists of four parts,
described as the ascending colon, the transverse colon, the descending
colon, and the sigmoid flexure, or sigmoid colon. The first three
divisions are named from the direction of the movement of materials
through them and the last from its shape, which is similar to that of
the Greek letter sigma (Σ).

The rectum is the last division of
the large intestine[pg 159] It is a nearly straight tube,
from six to eight inches in length, and connects with the external
surface of the body.

The general structure of the large intestine is similar to that of
the small intestine, and, like the small intestine, it is held in place
by the peritoneum. It differs from the small intestine, however, in its
lining of mucous membrane and in the arrangement of the muscular coat.
The mucous membrane presents a smooth appearance and has no villi, while
the longitudinal layer of the muscular coat is limited to three narrow
bands that extend along the greater length of the tube (Fig. 74). These
bands are shorter than the coats, and draw the large intestine into a
number of shallow pouches, by which it is readily distinguished from the
small intestine (Fig. 71).

Fig. 74—Section of large
intestine, showing the coats. 1. Serous coat. 2. Circular layer of
muscle. 3. Submucous coat. 4. Mucous membrane. 5. Muscular bands
extending lengthwise over the intestine.

Work of the Large Intestine.—The
large intestine serves as a receptacle for the materials from the small
intestine. The digestive fluids from the small intestine continue their
action here, and the dissolved materials also continue to be absorbed.
In these respects the work of the large intestine is similar to that of
the small intestine. It does, however, a work peculiar to itself in that
it collects and retains undigested food particles, together with other
wastes, and ejects them periodically from the canal.

Work of the Alimentary Muscles.—The
mechanical part of digestion is performed by the muscles that encircle
the food canal. Their uses, which have already been mentioned in
connection with the different organs of[pg 160] digestion, may be here
summarized: They supply the necessary force for masticating the food.
They propel the food through the canal. They mix the food with the
different juices. At certain places they partly or completely close the
passage until a digestive process is completed. They may even cause a
reverse movement of the food, as in vomiting. All of the alimentary
muscles, except those around the mouth, are involuntary. Their work is
of the greatest importance.

Other Purposes of the Digestive
Organs.—The digestive organs serve other important purposes besides
that of dissolving the foods. They provide favorable conditions for
passing the dissolved material into the blood. They dispose of such
portions of the foods as fail, in the digestive processes, to be reduced
to a liquid state. A considerable amount of waste material is also
separated from the blood by the glands of digestion (especially the
liver), and this is passed from the body with the undigested portions of
food. Then the food canal (stomach in particular) is a means of holding,
or storing, food which is awaiting the processes of digestion.
Considering the number of these purposes, the digestive organs are
remarkably simple, both in structure and in method of operation.

PRACTICAL WORK

Examine a dissectible model of the human abdomen (Fig. 75), noting
the form, location, and connection of the different organs. Find the
connection of the esophagus with the stomach, of the stomach with the
small intestine, and of the small intestine with the large
intestine.[pg 169] Sketch a general outline of the
cavity, and locate in this outline its chief organs.

Where it is desirable to learn something of the actual structure of
the digestive organs, the dissection of the abdomen of some small animal
is necessary. On account of unpleasant features likely to be associated
with such a dissection, however, this work is not recommended for
immature pupils.

Fig. 75—Model for demonstrating the abdomen and its
contents.

Dissection of the Abdomen.
(Optional)—For individual study, or for a small class, a half-grown cat
is perhaps the best available material. It should be killed with
chloroform, and then stretched, back downward, on a board, the feet
being secured to hold it in place.

The teacher should make a preliminary examination of the abdomen to
see that it is in a fit condition for class study. If the bladder is
unnaturally distended, its contents may be forced out by slight
pressure. The following materials will be needed during the dissection,
and should be kept near at hand: a sharp knife with a good point, a pair
of heavy scissors, a vessel of water, some cotton or a damp sponge, and
some fine cord. During the dissection the specimen should be kept as
clean as possible, and any escaping blood should be mopped up with the
cotton or the sponge. The dissection is best carried out by observing
the following order:

1. Cut through the abdominal wall in the center of the triangular
space where the ribs converge. From here cut a slit downward to the
lower portion of the abdomen, and sideward as far as convenient. Tack
the loosened abdominal walls to the board, and proceed to study the
exposed parts. Observe the muscles in the abdominal walls, and the fold
of the peritoneum which forms an
apron-like covering over the intestines.

2. Observe the position of the stomach, liver, spleen, and
intestines, and then, by pushing the intestines to one side, find the
kidneys and the bladder.

3. Study the liver with reference to its location, size, shape, and
color. On the under side, find the gall bladder, from which a small tube
leads to the small intestine. Observe the portal vein as it passes into
[pg 170] the liver. As the liver is filled
with blood, neither it nor its connecting blood vessels should be cut at
this time.

4. Trace out the continuity of the canal. Find the esophagus where it
penetrates the diaphragm and joins the stomach. Find next the union of
the stomach with the small intestine. Then, by carefully following the
coils of the small intestine, discover its union with the large
intestine.

5. Within the first coil of the small intestine, as it leaves the
stomach, find the pancreas. Note its
color, size, and branches. Find its connection with the small
intestine.

6. Beginning at the cut portion of the abdominal wall, lift the thin
lining of the peritoneum and carefully follow it toward the back and
central portion of the abdomen. Observe whether it extends back of or in
front of the kidneys, the aorta, and the inferior vena cava. Find where
it leaves the wall as a double
membrane, the mesentery, which
surrounds and holds in place the large and small intestines. Sketch a
coil of the intestine, showing the mesentery.

7. Find in the center of the coils of small intestine a long, slender
body having the appearance of a gland. This is the beginning of the thoracic duct and is called the receptacle of the chyle. From this the
thoracic duct rapidly narrows until it forms a tiny tube difficult to
trace in a small animal.

8. Cut away about two inches of the small intestine from the
remainder, having first tied the tube on the two sides of the section
removed. Split it open for a part of its length, and wash out its
contents. Observe its coats. Place it in a shallow vessel containing
water, and examine the mucous membrane with a lens to find the villi. Make a drawing of this section,
showing the coats.

9. Study the connection of the small intestine with the large. Split
them open at the place of union, wash out the contents, and examine the
ileo-cæcal valve.

10. Observe the size, shape, and position of the kidneys. Do they lie
in front of or back of the peritoneum? Do they lie exactly opposite each
other? Note the connection of each kidney with the aorta and the
inferior vena cava by the renal artery and the renal vein. Find a
slender tube, the ureter, running
from each kidney to the bladder. Do the ureters connect with the top or
with the base of the bladder? Show by a sketch the connection of the
kidneys with the large blood vessels and the bladder.

[pg 171]To
demonstrate the Teeth.—Procure from the dentist a collection of
different kinds of teeth, both sound and decayed.

(a) Examine external surfaces of
different kinds of teeth, noting general shape, cutting or grinding
surfaces, etc. Make a drawing of an incisor and also of a molar.

(b) After soaking some of the
teeth for a couple of days in warm water saw one of them in two
lengthwise, and another in two crosswise, and smooth the cut surfaces
with fine emery or sand paper. Examine both kinds of sections, noting
arrangement and extent of dentine, enamel, and pulp. Make drawings.

(c) Examine a decayed tooth. Which
substance of the tooth appears to decay most readily? Why is it
necessary to cut away a part of the tooth before filling?

(d) Test the effect of acids upon
the teeth by leaving a tooth over night in a mixture of one part
hydrochloric acid to four parts water, and by leaving a second tooth for
a couple of days in strong vinegar. Examine the teeth exposed to the
action of acids, noting results.

To show the Importance of
Mastication.—Fill two tumblers each half full of water. Into one
put a lump of rock salt. Into the other place an equal amount of salt
that has been finely pulverized. Which dissolves first and why?

To illustrate Acid and Alkaline
Reactions.—To a tumbler half full of water add a teaspoonful of
hydrochloric or other acid, as vinegar. To a second tumbler half full of
water add an equal amount of cooking soda. Taste each liquid, noting the
sour taste of the acid, and the alkaline taste of the soda. Hold a piece
of red litmus paper in the soda solution, noting that it is turned blue.
Then hold a piece of blue litmus paper in the acid solution, noting that
it is turned red. Add acid to the soda solution, and soda to the acid
solution, until the conditions are reversed, testing with the red and
blue litmus papers.

Hold, for a minute or longer, a narrow strip of red litmus paper in
the mouth, noting any change in the color of the paper. Repeat, using
blue litmus paper. What effect, if any, has the saliva upon the color of
the papers? Has the mouth an acid or an alkaline reaction?

To show the Action of Saliva on
Starch.—1 (Optional). Prepare starch paste by mixing half a
teaspoonful of starch in half a pint of water and heating the mixture to
boiling. Place some of this in a test tube and thin it by adding more
water. Then add a small drop of[pg 172] iodine solution (page 136) to the
solution of starch. It should turn a deep blue color. This is the test
for starch.

Now collect from the mouth, in a clean test tube, two or three
teaspoonfuls of saliva. Add portions of this to small amounts of fresh
starch solution in two test tubes. Let the tubes stand for five or ten
minutes surrounded by water having about the temperature of the body.
Test for changes that have occurred as follows:

(a) To one tube add a little of
the iodine solution. If it does not turn blue, it shows that the starch
has been converted into some other substance by the saliva, (b) To the other tube add a few drops of a
very dilute solution of copper sulphate. Then add sodium (or potassium)
hydroxide, a few drops at a time, until the precipitate which first
forms dissolves and turns a deep blue. Then gradually heat the upper
portion of the liquid to boiling. If it turns an orange or yellowish red
color, the presence of a form of sugar (maltose or dextrose) is proved.
See page 136.

2. Hold some powdered starch in the mouth until it completely
dissolves and observe that it gradually acquires a sweetish taste. This
shows the change of starch into sugar.

To illustrate the Action of the Gastric
Juice.—Add to a tumbler two thirds full of water as much scale
pepsin (obtained from a drug store) as will stay on the end of the large
blade of a penknife. Then add enough hydrochloric acid to give a
slightly sour taste. Place in the artificial gastric juice thus prepared
some boiled white of egg which has been finely divided by pressing it
through a piece of wire gauze. Also drop in a single large lump. Keep in
a warm place (about the temperature of the body) for several hours or a
day, examining from time to time. What is the general effect of the
artificial gastric juice upon the egg?

To illustrate Effect of Alcohol upon
Gastric Digestion.—Prepare a tumbler half full of artificial
gastric juice as in the above experiment, and add 10 cubic centimeters
of this to each of six clean test tubes bearing labels. To five of the
tubes add alcohol from a burette as follows: (1) .5 c.c., (2) 1 c.c.,
(3) 1.5 c.c., (4) 2 c.c., and (5) 3 c.c., leaving one tube without
alcohol. Now add to each tube about 1/4 gram of finely divided white of
egg from the experiment above, and place all of the tubes in a beaker
half full of water. Keep the water a little above the temperature of the
body for several hours, examining the tubes at intervals to note the
progress of digestion. Inferences.