*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 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 role. 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 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.
[Fig. 1]
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.
[Fig. 2]
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 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, 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 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?
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?
PRACTICAL WORK
*Observation on the Tissues.*--Examine with care the structures in the
entire leg of a chicken, squirrel, rabbit, or other small animal used for
food. Observe, first of all, the external covering, consisting of cuticle
and hair, claws, scales, or feathers, according to the specimen. These are
similar in structure, and they form the epidermis, which is one kind of
_epithelial_ tissue. With a sharp knife lay open the skin and observe that
it is attached to the parts underneath by thin, but tough, threads and
sheaths. These represent a variety of _connective_ tissue. The reddish
material which forms the greater portion of the specimen is a variety of
_muscular_ tissue, and its divisions are called muscles. With a blunt
instrument, separate the muscles, by tearing apart the connective tissue
binding them together, and find the glistening white strips of connective
tissue (tendons) which attach them to the bones. Find near the central
part of the leg a soft, white cord (a nerve) which represents one variety
of _nervous_ tissue. The bones, which may now be examined, form the
_osseous_ tissue. At the ends of the bones will be found a layer of
smooth, white material which represents one kind of _cartilaginous_
tissue. The _adipose_, or fatty, tissue, which is found under the skin and
between the other tissues, is easily recognized.
*Relation of the Tissues to the Organs.*--Observe in the specimen just
studied the relation of the different tissues to the organ as a whole
(regarding the leg as an organ), _i.e._, show how each of the tissues aids
in the work which the organ accomplishes. Show in particular how the
muscles supply the foot with motion, by tracing out the tendons that
connect them with the toes. Pull on the different tendons, noting the
effect upon the different parts of the foot.