_Demonstration._--Review to show that the human body is a
complex of cells.
_Laboratory demonstration_ by means of (_a_) human skeleton
and (_b_) manikin to show the position and gross structure
of the chief organs of man.
Man and his Environment.--In the last chapter we saw that one factor in the
improvement of man lies in giving him better surroundings. It will be the
purpose of the following chapters to show how man is fitted to live in the
environment in which he is placed. He comes in contact with air, light,
water, soil, food, and shelter which make his somewhat artificial
environment; he must adapt himself to get the best he can out of this
environment.
The Needs of Living Things.--We have already found that the primary needs
of plants and animals are the same. They both need food, they both need to
digest their food and to have it circulate in a fluid form to the cells
where it will be used. They both need oxygen so as to release the energy
locked up in their food. And they both need to reproduce so that their kind
may be continued on the earth. What is true of plants and other animals is
true of man.
The Needs of Simple and Complex Animals the Same.--The simplest animal, a
single cell, has the same needs as the most complex. The _cell_ paramoecium
feeds, digests, oxidizes its food, and releases energy. The _cells_ of the
human body built up into tissues have the same needs and perform the same
functions as the paramoecium. It is the _cells_ of the body working
together in groups as tissues and organs that make the complicated actions
of man possible. Division of labor has arisen because of the complex needs
and work of the organism.
[Illustration: The human body seen from the side in longitudinal section.]
The Human Body a Machine.--In all animals, and the human animal is no
exception, the body has been likened to a machine in that it turns over the
_latent_ or potential energy stored up in food into _kinetic_ energy
(mechanical work and heat), which is manifested when we perform work. One
great difference exists between an engine and the human body. The engine
uses fuel unlike the substance out of which it is made. The human body, on
the other hand, uses for fuel the same substances out of which it is
formed; it may, indeed, use part of its own substance for food. It must as
well do more than purely mechanical work. The human organism must be so
delicately adjusted to its surroundings that it will react in a ready
manner to stimuli from without; it must be able to utilize its fuel (food)
in the most economical manner; it must be fitted with machinery for
transforming the energy received from food into various kinds of work; it
must properly provide the machine with oxygen so that the fuel will be
oxidized, and the products of oxidation must be carried away, as well as
other waste materials which might harm the effectiveness of the machine.
Most important of all, the human machine must be able to repair itself.
In order to understand better this complicated machine, the human body, let
us briefly examine the structure of its parts and thus get a better idea of
the interrelation of these parts and of their functions.
The Skin.--Covering the body is a protective structure called the skin.
Covered on the outside with dead cells, yet it is provided with delicate
sense organs, which give us perception of touch, taste, smell, pressure,
and temperature. It also aids in getting wastes out of the body by means of
its sweat glands and plays an important part in equalizing the temperature
of the body.
[Illustration: Skeleton of a man. _CR._, cranium; _CL._, clavicle; _ST._,
sternum; _H._, humerus; _V.C._, vertebral column; _R._, radius; _U._, ulna;
_P._, pelvic girdle; _C._, carpals; _M._, metacarpals; _Ph._, phalanges;
_F._, femur; _Fi._, fibula; _T._, tibia; _Tar._, tarsals; _MT._,
metatarsals.]
Bones and Muscles.--The body is built around a framework of bones. These
bones, which are bound together by tough _ligaments_, fall naturally into
two great groups, the bones of the body proper, vertebral column, ribs,
breast bone, and skull, which form the _axial_ skeleton, and the
appendages, two sets of bones which form the framework of the arms and
legs, which with the bones which attach them to the axial skeleton form the
_appendicular_ skeleton.
To the bones are attached the muscles of the body. Movement is accomplished
by contraction of muscles, which are attached so as to cause the bones to
act as levers. Bones also protect the nervous system and other delicate
organs. They also help to give form and rigidity to the body.
[Illustration: Diagram showing action of biceps muscle. _a_, contracted;
_b_, extended; _h_, humerus; _s_, scapula.]
Hygiene of Muscles and Bones.--Young people especially need to know how to
prevent certain defects which are largely the result of bad habits of
posture. Standing erect is an example of a good habit, round shoulders a
bad habit of this sort. The habit of a wrong position of bones and muscles
once formed is very hard to correct. This can best be done by certain
corrective exercises at home or in the gymnasium.
Round shoulders is most common among people whose occupation causes them to
stoop. Drawing, writing, and a wrong position when at one's desk are among
the causes. Exercises which strengthen the back muscles and cause the head
to be kept erect are helpful in forming the habit of erect carriage.
Slight curvature of the spine either backward or forward is helped most by
exercises which tend to straighten the body, such as stretching up with the
hands above the head. Lateral curvature of the spine, too often caused by a
"hunched-up" position at the school desk, may also be corrected by
exercises which tend to lengthen the spinal column.
[Illustration: Three classes of levers in the human body; bones and muscles
act together. _A_, a lever of the first class; _B_, a lever of the second
class; _C_, a lever of the third class.]
It is the duty of every girl and boy to have good posture and erect
carriage, not only because of the better state of health which comes with
it, but also because one's self-respect demands that each one of us makes
the best of the gifts that nature has given us. An erect head, straight
shoulders, and elastic carriage go far toward making their owner both liked
and respected.
[Illustration: Bad posture in the schoolroom may cause permanent injury to
the spine.]
Other Body Structures.--In spaces between the muscles are found various
other structures,--blood vessels, which carry blood to and from the great
pumping station, the heart, and thence to all parts of the body; connective
tissue, which holds groups of muscle or other cells together; fat cells,
scattered in various parts of the body; various gland cells, which
manufacture enzymes; and the cells of the nervous system, which aid in
directing the body parts.
Body Cavity.--Within the body is a cavity, which in life is almost
completely filled with various organs. A thin wall of muscle called the
_diaphragm_ divides the body cavity into two unequal spaces. In the upper
space are found the _heart_ and _lungs_, in the lower, the digestive tract
with its glands, the _liver_, _kidneys_, and other structures (see page 267).
Digestion, Absorption, and Excretion.--Running through the body is a food
tube in which undigested food is placed and from which digested or liquid
food is absorbed into the blood so that the cells of the various organs
which do the work may receive food. Emptying into this food tube are
various groups of gland cells, which pour digestive fluids over the solid
foods, thus aiding in changing them to liquids. Solid wastes are passed out
through the posterior end of the food tube, while liquid wastes are
excreted by means of glands called _kidneys_.
Work done by Cells.--Food, prepared in the digestive tract, and oxygen from
the lungs are taken by the blood to the cells. Bathed in liquid food, the
cells do their work; they promote the oxidization of food and the exchange
of carbon dioxide for oxygen in the blood, while other wastes of the cells
are given off, to pass eventually through the kidneys and out of the body.
The Nervous System.--The smooth working of the bodily machine is due to
another set of structures which direct the working of the parts so that
they will act in unison. This director is the nervous system. We have seen
that, in the simplest of animals, one cell performs the functions necessary
to its existence. In the more complex animals, where groups of cells form
tissues, each having a different function, a nervous system is developed.
_The functions of the human nervous system are:_ (1) _the providing of man
with sensation, by means of which he gets in touch with the world about
him;_ (2) _the connecting of organs in different parts of the body so that
they act as a united and harmonious whole;_ (3) _the giving to the human
being a will, a provision for thought._ Cooperation in word and deed is the
end attained. We are all familiar with examples of the cooperation of
organs. You see food; the thought comes that it is good to eat; you reach
out, take it, raise it to the mouth; the jaws move in response to your
will; the food is chewed and swallowed. While digestion and absorption of
the food are taking place, the nervous system is still in control. The
nervous system also regulates pumping of blood over the body, respiration,
secretion of glands, and, indeed, every bodily function. Man is the highest
of all animals because of the extreme development of the nervous system.
Man is the thinking animal, and as such is master of the earth.