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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

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.