d 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.

[Illustration: FIG. 16.--Diagram illustrating the paths of association.]

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
continua