ftest 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 develope