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Development Of Nervous System Through Use

IMPORTANCE OF STIMULUS AND RESPONSE.--Like all other tissues of the
body, the nerve cells and fibers are developed by judicious use. The
sensory and association centers require the constant stimulus of nerve
currents running in from the various end-organs, and the motor centers
require the constant stimulus of currents running from them out to the
muscles. In other words, the conditions upon which both motor and
sensory development depend are: (1) A rich environment of sights and
sounds and tastes and smells, and everything else which serves as proper
stimulus to the sense organs, and to every form of intellectual and
social interest; and (2) no less important, an opportunity for the
freest and most complete forms of response and motor activity.

An illustration of the effects of the lack of sensory stimuli on the
cortex is well shown in the case of Laura Bridgman, whose brain was
studied by Professor Donaldson after her death. Laura Bridgman was born
a normal child, and developed as other children do up to the age of
nearly three years. At this time, through an attack of scarlet fever,
she lost her hearing completely and also the sight of her left eye. Her
right eye was so badly affected that she could see but little; and it,
too, became entirely blind when she was eight. She lived in this
condition until she was sixty years old, when she died. Professor
Donaldson submitted the cortex of her brain to a most careful
examination, also comparing the corresponding areas on the two
hemispheres with each other. He found that as a whole the cortex was
thinner than in the case of normal individuals. He found also that the
cortical area connected with the left eye--namely, the right occipital
region--was much thinner than that for the right eye, which had retained
its sight longer than the other. He says: It is interesting to notice
that those parts of the cortex which, according to the current view,
were associated with the defective sense organs were also particularly
thin. The cause of this thinness was found to be due, at least in part,
to the small size of the nerve cells there present. Not only were the
large and medium-sized cells smaller, but the impression made on the
observer was that they were also less numerous than in the normal

EFFECT OF SENSORY STIMULI.--No doubt if we could examine the brain of a
person who has grown up in an environment rich in stimuli to the eye,
where nature, earth, and sky have presented a changing panorama of color
and form to attract the eye; where all the sounds of nature, from the
chirp of the insect to the roar of the waves and the murmur of the
breeze, and from the softest tones of the voice to the mightiest sweep
of the great orchestra, have challenged the ear; where many and varied
odors and perfumes have assailed the nostrils; where a great range of
tastes have tempted the palate; where many varieties of touch and
temperature sensations have been experienced--no doubt if we could
examine such a brain we should find the sensory areas of the cortex
excelling in thickness because its cells were well developed and full
sized from the currents which had been pouring into them from the
outside world. On the other hand, if we could examine a cortex which had
lacked any one of these stimuli, we should find some area in it
undeveloped because of this deficiency. Its owner therefore possesses
but the fraction of a brain, and would in a corresponding degree find
his mind incomplete.

NECESSITY FOR MOTOR ACTIVITY.--Likewise in the case of the motor areas.
Pity the boy or girl who has been deprived of the opportunity to use
every muscle to the fullest extent in the unrestricted plays and games
of childhood. For where such activities are not wide in their scope,
there some areas of the cortex will remain undeveloped, because unused,
and the person will be handicapped later in his life from lack of skill
in the activities depending on these centers. Halleck says in this
connection: If we could examine the developing motor region with a
microscope of sufficient magnifying power, it is conceivable that we
might learn wherein the modification due to exercise consists. We might
also, under such conditions, be able to say, 'This is the motor region
of a piano player; the modifications here correspond precisely to those
necessary for controlling such movements of the hand.' Or, 'This is the
motor tract of a blacksmith; this, of an engraver; and these must be the
cells which govern the vocal organs of an orator.' Whether or not the
microscope will ever reveal such things to us, there is no doubt that
the conditions suggested exist, and that back of every inefficient and
awkward attempt at physical control lies a motor area with its cells
undeveloped by use. No wonder that our processes of learning physical
adjustment and control are slow, for they are a growth in the brain
rather than a simple learning how.

The training of the nervous system consists finally, then, in the
development and cooerdination of the neurones of which it is composed. We
have seen that the sensory cells are to be developed by the sensory
stimuli pouring in upon them, and the motor cells by the motor impulses
which they send out to the muscles. The sensory and the motor fibers
likewise, being an outgrowth of their respective cells, find their
development in carrying the impulses which result in sensation and
movement. Thus it is seen that the neurone is, in its development as in
its work, a unit.

DEVELOPMENT OF THE ASSOCIATION CENTERS.--To this simpler type of sensory
and motor development which we have been considering, we must add that
which comes from the more complex mental processes, such as memory,
thought, and imagination. For it is in connection with these that the
association fibers are developed, and the brain areas so connected that
they can work together as a unit. A simple illustration will enable us
to see more clearly how the nervous mechanism acts to bring this about.

Suppose that I am walking along a country road deeply engaged in
meditation, and that I come to a puddle of water in my pathway. I may
turn aside and avoid the obstruction without my attention being called
to it, and without interruption of my train of thought. The act has been
automatic. In this case the nerve current has passed from the eye (S)
over an afferent fiber to a sensory center (s) in the nervous system
below the cortex; from there it has been forwarded to a motor center
(m) in the same region, and on out over a motor fiber to the proper
muscles (M), which are to execute the required act. The act having
been completed, the sensory nerves connected with the muscles employed
report the fact back that the work is done, thus completing the circuit.
This event may be taken as an illustration of literally thousands of
acts which we perform daily without the intervention of consciousness,
and hence without involving the hemispheres.

If, however, instead of avoiding the puddle unconsciously, I do so from
consideration of the danger of wet feet and the disagreeableness of
soiled shoes and the ridiculous appearance I shall make, then the
current cannot take the short circuit, but must pass on up to the
cortex. Here it awakens consciousness to take notice of the obstruction,
and calls forth the images which aid in directing the necessary
movements. This simple illustration may be greatly complicated,
substituting for it one of the more complex problems which are
continually presenting themselves to us for solution, or the associated
trains of thought that are constantly occupying our minds. But the truth
of the illustration still holds. Whether in the simple or the complex
act, there is always a forward passing of the nerve current through the
sensory and thought centers, and on out through the motor centers to the
organs which are to be concerned in the motor response.

THE FACTORS INVOLVED IN A SIMPLE ACTION.--Thus it will be seen that in
the simplest act which can be considered there are the following
factors: (1) The stimulus which acts on the end-organ; (2) the ingoing
current over an afferent nerve; (3) the sensory or interpreting cells;
(4) the fibers connecting the sensory with a motor center; (5) the motor
cells; (6) the efferent nerve to carry the direction for the movement
outward to the muscle; (7) the motor response; and, finally, (8) the
report back that the act has been performed. With this in mind it fairly
bewilders one to think of the marvelous complexity of the work that is
going on in our nervous mechanism every moment of our life, even without
considering the higher thought processes at all. How, with these added,
the resulting complexity all works out into beautiful harmony is indeed
beyond comprehension.

Next: Education And The Training Of The Nervous System

Previous: Factors Determining The Efficiency Of The Nervous System

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