_Demonstration._--Sensory motor reactions.
_Demonstration._--Nervous system. Models and frog
dissections.
_Demonstration._--Neurones under compound microscope
(optional).
_Demonstration._--Reflex acts are unconscious acts: show how
conscious acts may become habitual.
_Home exercise_ in habit forming.
_The senses.--Home exercises._--(1) To determine areas most
sensitive to touch. (2) To determine or map out hot and cold
spots on an area on the wrist. (3) To determine functions of
different areas on tongue.
_Demonstration._--Show how eye defects are tested.
_Laboratory summary._--The effects of alcohol on the nervous
system.
The Body a Self-directed Machine.--Throughout the preceding chapters the
body has been likened to an engine, which, while burning its fuel, food,
has done work. If we were to carry our comparison further, however, the
simile ceases. For the engineer runs the engine, while the bodily machine
is self-directive.
Moreover, most of the acts we perform during a day's work are results of
the automatic working of this bodily machine. The heart pumps; the blood
circulates its load of food, oxygen, and wastes; the movements of breathing
are performed; the thousand and one complicated acts that go on every day
within the body are _seemingly_ undirected.
[Illustration: The central nervous system.]
Automatic Activity.--In addition to this, numbers of other of our daily
acts are not thought about. If we are well-regulated body machines, we get
up in the morning, automatically wash, clean our teeth, dress, go to the
toilet, get our breakfast, walk to school, even perform such complicated
processes as that of writing, without _thinking_ about or _directing_ the
machine. In these respects we have become creatures of habit. Certain acts
which once we might have learned consciously, have become automatic.
But once at school, if we are really making good in our work in the
classroom, we begin a higher control of our bodily functions. Automatic
control acts no longer, and sensation is not the only guide--for we now
begin to make _conscious choice_; we weigh this matter against another,--in
short, we _think_.
Parts of the Nervous System.--This wonderful self-directive apparatus
placed within us, which is in part under control of our will, is known as
the nervous system. In the vertebrate animals, including man, it consists
of two divisions. One includes the brain, spinal cord, the cranial and
spinal nerves, which together make up the _cerebro-spinal nervous system_.
The other division is called the _sympathetic nervous system_ and has to do
with those bodily functions which are beyond our control. Every group of
cells in the body that has work to do (excepting the floating cells of the
blood) is directly influenced by these nerves. Our bodily comfort is
dependent upon their directive work. The organs which put us in touch with
our surroundings are naturally at the _surface_ of the body. Small
collections of nerve cells, called _ganglia_, are found in all parts of the
body. These nerve centers are connected, to a greater or less degree, with
the surface of the body by the nerves, which serve as pathways between the
end organs of touch, sight, taste, etc., and the centers in the brain or
spinal cord. Thus sensation is obtained.
Sensations and Reactions.--We have already seen that simpler forms of life
perform certain acts because certain outside forces acting upon them cause
them to _react_ to the stimulus from without. The one-celled animal
responds to the presence of food, to heat, to oxygen, to other conditions
in its surroundings. An earthworm is repelled by light, is attracted by
food. All animals, including man, are put in touch with their surroundings
by what we call the organs of sensation. The senses of man, besides those
we commonly know as those of sight, hearing, taste, smell, and touch, are
those of temperature, pressure, and pain. It is obvious that such organs,
if they are to be of use to an animal, must be at the outside of the body.
Thus we find eyes and ears in the head, and taste cells in the mouth, while
other cells in the nose perceive odors, and still others in the skin are
sensitive to heat or cold, pressure or pain.
But this is not all. Strangely enough, we do not see with our eyes or taste
with our taste cells. These organs receive the sensations, and by means of
a complicated system of greatly elongated cell structures, the message is
sent inward, relayed by other elongated cells until the sensory message
reaches an inner station, in the central nervous system. We see and hear
and smell in our brain. Let us next examine the structure of the nerve
cells or _neurons_ part of which serve as pathways for these messages.
[Illustration: Diagram of a neuron or nerve unit.]
Neurones.--A nerve cell, like other cells in the body, is a mass of
protoplasm containing a nucleus. But the body of the nerve cell is usually
rather irregular in shape, and distinguished from most other cells by
possessing several delicate, branched protoplasmic projections called
_dendrites_. One of these processes, the axon, is much longer than the
others and ends in a muscle or organ of sensation. The axon forms the
pathway over which nervous impulses travel to and from the nerve centers.
A nerve consists of a bundle of such tiny axons, bound together by
connective tissue. As a nerve ganglia is a center of activity in the
nervous system, so a cell body is a center of activity which may send an
impulse over this thin strand of protoplasm (the axon) prolonged many
hundreds of thousands of times the length of the cell. Some neurones in the
human body, although visible only under the compound microscope, give rise
to axons several feet in length.
Because some bundles of axons originate in organs that receive sensations
and send those sensations to the central nervous system, they are called
_sensory nerves_. Other axons originate in the central nervous system and
pass outward as nerves producing movement of muscles. These are called
_motor nerves_.
The Brain of Man.--In man, the central nervous system consists of a brain
and spinal cord inclosed in a bony case. From the brain, twelve pairs of
nerves are given off; thirty-one pairs more leave the spinal cord. The
brain has three divisions. The _cerebrum_ makes up the largest part. In
this respect it differs from the cerebrum of the frog and other
vertebrates. It is divided into two lobes, the _hemispheres_, which are
connected with each other by a broad band of nerve fibers. The outer
surface of the cerebrum is thrown into folds or _convolutions_ which give a
large surface, the cell bodies of the neurons being found in this part of
the cerebrum. Holding the cell bodies and fibers in place is a kind of
connective tissue. The inner part (white in color) is composed largely of
fibers which pass to other parts of the brain and down into the spinal
cord. Under the cerebrum, and dorsal to it, lies the little brain, or
_cerebellum_. The two sides of the cerebellum are connected by a band of
nerve fibers which run around into the lower hindbrain or _medulla_. This
band of fibers is called the _pons_. The medulla is, in structure, part of
the spinal cord, and is made up largely of fibers running longitudinally.
The Sympathetic Nervous System.--Connected with the central nervous system
is that part of the nervous apparatus that controls the muscles of the
digestive tract and blood vessels, the secretions of gland cells, and all
functions which have to do with life processes in the body. This is called
the sympathetic nervous system.
Functions of the Parts of the Central Nervous System of the Frog.--From
careful study of living frogs, birds, and some mammals we have learned much
of what we know of the functions of the parts of the central nervous system
in man.
It has been found that if the entire brain of a frog is destroyed and
separated from the spinal cord, "the frog will continue to live, but with a
very peculiarly modified activity." It does not appear to breathe, nor does
it swallow. It will not move or croak, but if acid is placed upon the skin
so as to irritate it, the legs make movements to push away and to clean off
the irritating substance. The spinal cord is thus shown to be a center for
defensive movements. If the cerebrum is separated from the rest of the
nervous system, the frog seems to act a little differently from the normal
animal. It jumps when touched, and swims when placed in water. It will
croak when stroked, or swallow if food be placed in its mouth. But it
manifests no hunger or fear, and is in every sense a machine which will
perform certain actions after certain stimulations. Its movements are
automatic. If now we watch the movements of a frog which has the brain
uninjured in any way, we find that it acts _spontaneously_. It tries to
escape when caught. It feels hungry and seeks food. It is capable of
voluntary action. It acts like a normal individual.
[Illustration: Diagram to show the parts of the brain and action of the
different parts of the brain.]
Functions of the Cerebrum.--In general, the functions of the different
parts of the brain in man agree with those functions we have already
observed in the frog. The cerebrum has to do with conscious activity; that
is, thought. It presides over what we call our thoughts, our will, and our
sensations. A large part of the area of the outer layer of the cerebrum
seems to be given over to some one of the different functions of speech,
hearing, sight, touch, movements of bodily parts. The movement of the
smallest part of the body appears to have its definite localized center in
the cerebrum. Experiments have been performed on monkeys, and these,
together with observations made on persons who had lost the power of
movement of certain parts of the body, and who, after death, were found to
have had diseases localized in certain parts of the cerebrum, have given to
us our knowledge on this subject.
[Illustration: Diagram of the nerve path of a simple reflex action.]
Reflex Actions; their Meaning.--If through disease or for other reasons the
cerebrum does not function, no will power is exerted, nor are intelligent
acts performed. All acts performed in such a state are known as _reflex
actions_. The involuntary brushing of a fly from the face, or the attempt
to move away from the source of annoyance when tickled with a feather, are
examples of reflexes. In a reflex act, a person does not think before
acting. The nervous impulse comes from the outside to cells that are not in
the cerebrum. The message is short-circuited back to the surface by motor
nerves, without ever having reached the thinking centers. The nerve cells
which take charge of such acts are located in the cerebellum or spinal
cord.
Automatic Acts.--Some acts, however, are learned by conscious thought, as
writing, walking, running, or swimming. Later in life, however, these
activities become automatic. The actual performance of the action is then
taken up by the cerebellum, medulla, and spinal ganglia. Thus the thinking
portion of the brain is relieved of part of its work.
Bundles of Habits.--It is surprising how little real thinking we do during
a day, for most of our acts are habitual. Habit takes care of our dressing,
our bathing, our care of the body organs, our methods of eating; even our
movements in walking and the kind of hand we write are matters of habit
forming. We are bundles of habits, be they good ones or bad ones.
Habit Formation.--The training of the different areas in the cerebrum to do
their work well is the object of education. When we learned to write, we
exerted conscious effort in order to make the letters. Now the act of
forming the letters is done without thought. By training, the act has
become automatic. In the beginning, a process may take much thought and
many trials before we are able to complete it. After a little practice, the
same process may become almost automatic. We have formed a habit. Habits
are really acquired reflex actions. They are the result of nature's method
of training. The conscious part of the brain has trained the cerebellum or
spinal cord to do certain things that, at first, were taken charge of by
the cerebrum.
Importance of Forming Right Habits.--Among the habits early to be acquired
are the habits of studying properly, of concentrating the mind, of learning
self-control, and, above all, of contentment. Get the most out of the world
about you. Remember that the immediate effect in the study of some subjects
in school may not be great, but the cultivation of correct methods of
thinking may be of the greatest importance later in life. The man or woman
who has learned how to concentrate on a problem, how to weigh all sides
with an unbiased mind, and then to decide on what they believe to be best
and right are the efficient and happy ones of their generation.
"The hell to be endured hereafter, of which theology tells,
is no worse than the hell we make for ourselves in this
world by habitually fashioning our characters in the wrong
way. Could the young but realize how soon they will become
mere walking bundles of habits, they would give more heed to
their conduct while in the plastic state. We are spinning
our own fates, good or evil, and never to be undone. Every
smallest stroke of virtue or of vice leaves its
never-so-little scar. The drunken Rip Van Winkle, in
Jefferson's play, excuses himself for every fresh
dereliction by saying, 'I won't count this time!' Well! he
may not count it, and a kind Heaven may not count it; but it
is being counted none the less. Down among his nerve cells
and fibers the molecules are counting it, registering and
storing it up to be used against him when the next
temptation comes. Nothing we ever do is, in strict
scientific literalness, wiped out. Of course this has its
good side as well as its bad one. As we become permanent
drunkards by so many separate drinks, so we become saints in
the moral, and authorities in the practical and scientific,
spheres by so many separate acts and hours of work. Let no
youth have any anxiety about the upshot of his education,
whatever the line of it may be. If he keep faithfully busy
each hour of the working day, he may safely leave the final
result to itself. He can with perfect certainty count on
waking up some fine morning, to find himself one of the
competent ones of his generation, in whatever pursuit he may
have singled out."--JAMES, _Psychology_.
Some Rules for Forming Good Habits.--Professor Horne gives several rules
for making good or breaking bad habits. They are: "First, _act on every
opportunity_. Second, _make a strong start_. Third, _allow no exception_.
Fourth, _for the bad habit establish a good one_. Fifth, summoning all the
man within, _use effort of will_." Why not try these out in forming some
good habit? You will find them effective.
[Illustration: The effect of fatigue on nerve cells. _a_, healthy brain
cell; _b_, fatigued brain cell.]
Necessity of Food, Fresh Air, and Rest.--The nerve cells, like all other
cells in the body, are continually wasting away and being rebuilt.
Oxidation of food material is more rapid when we do mental work. The cells
of the brain, like muscle cells, are not only capable of fatigue, but show
this in changes of form and of contents. _Food_ brought to them in the
blood, plenty of _fresh air_, especially when engaged in active brain work,
and _rest_ at proper times, are essential in keeping the nervous system in
condition. One of the best methods of resting the brain cells is a change
of occupation. Tennis, golf, baseball, and other outdoor sports combine
muscular exercise with brain activity of a different sort from that of
business or school work. But change of occupation will not rest exhausted
neurones. For this, sleep is necessary. Especially is sleep an important
factor in the health of the nervous system of growing children.
Necessity of Sleep.--Most brain cells attain their growth early in life.
Changes occur, however, until some time after the school age. Ten hours of
sleep should be allowed for a child, and at least eight hours for an adult.
At this time, only, do the brain cells have opportunity to rest and store
food and energy for their working period.
Sleep is one way in which all cells in the body, and particularly those of
the nervous system, get their rest. The nervous system, by far the most
delicate and hardest-worked set of tissues in the body, needs rest more
than do other tissues, for its work directing the body only ends with sleep
or unconsciousness. The afternoon nap, snatched by the brain worker, gives
him renewed energy for his evening's work. It is not hard application to a
task that wearies the brain; it is _continuous_ work without rest.