center brings loss of auditory knowledge.

Injury to the retina or optic nerve, occurring early in life, results
in an under-development of the cortex in the occipital lobe. The nerve
cells remain small and their dendrites few and meager, because they
have not received their normal amount of exercise through stimulation
from the eye.

Exercise, then, has the same general effect on neurones that it has on
muscles; it causes them to grow and it probably also improves their
internal condition so that they act more readily and more strongly.
The growth, in the cortex, of dendrites and of the end-brushes of
axons that interlace with the dendrites, must improve the synapses
between one neurone and another, and thus make better conduction paths
between one part of the cortex and another, and also between the
cortex and the lower sensory and motor centers.

The law of exercise has thus a very definite meaning when {415}
translated into neural terms. It means that the synapses between
stimulus and response are so improved, when traversed by nerve
currents in the making of a reaction, that nerve currents can get
across them more easily the next time.

[Illustration: Fig. 63.--The law of exercise in terms of synapse. A
nerve current is supposed to pass along this pair of neurones in the
direction of the arrow. Every time it passes, it exercises the
end-brush and dendrites at the synapse (for the "passage of a nerve
current" really means activity on the part of the neurones through
which it passes), and the after-effect of this exercise is growth of
the exercised parts, and consequent improvement of the synapse as a
linkage between one neurone and the other. Repeated exercise may
probably bring a synapse from a very loose condition to a state of
close interweaving and excellent power of transmitting the nerve
current.]

The more a synapse is used, the better synapse it becomes, and the
better linkage it provides between some stimulus and some response.
The cortex is the place where linkages are made in the process of
learning, and it is there also that forgetting, or atrophy, takes
place through disuse. Exercise makes a synapse closer, disuse lets it
relapse into a loose and poorly conducting state.

The law of combination, also, is readily translated into {416} neural
terms. The "pre-existing loose linkages" which it assumed to exist
undoubtedly do exist in the form of "association fibers" extending in
vast numbers fro