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 cortex."
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