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
coördination 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.