OF FUNCTION IN THE NERVOUS SYSTEM

DIVISION OF LABOR.--Division of labor is the law in the organic world as
in the industrial. Animals of the lowest type, such as the amoeba, do
not have separate organs for respiration, digestion, assimilation,
elimination, etc., the one tissue performing all of these functions. But
in the higher forms each organ not only has its own specific work, but
even within the same organ each part has its own particular function
assigned. Thus we have seen that the two parts of the neurone probably
perform different functions, the cells generating energy and the fibers
transmitting it.

It will not seem strange, then, that there is also a division of labor
in the cellular matter itself in the nervous system. For example, the
little masses of ganglia which are distributed at intervals along the
nerves are probably for the purpose of reenforcing the nerve current,
much as the battery cells in the local telegraph office reenforce the
current from the central office. The cellular matter in the spinal cord
and lower parts of the brain has a very important work to perform in
receiving messages from the senses and responding to them in directing
the simpler reflex acts and movements which we learn to execute without
our consciousness being called upon, thus leaving the mind free from
these petty things to busy itself in higher ways. The cellular matter of
the cortex performs the highest functions of all, for through its
activity we have consciousness.

[Illustration: FIG. 13.--Side view of left hemisphere of human brain,
showing the principal localized areas.]

The gray matter of the cerebellum, the medulla, and the cord may receive
impressions from the senses and respond to them with movements, but
their response is in all cases wholly automatic and unconscious. A
person whose hemispheres had been injured in such a way as to interfere
with the activity of the cortex might still continue to perform most if
not all of the habitual movements of his life, but they would be
mechanical and not intelligent. He would lack all higher consciousness.
It is through the activity of this thin covering of cellular matter of
the cerebrum, the _cortex_, that our minds operate; here are received
stimuli from the different senses, and here sensations are experienced.
Here all our movements which are consciously directed have their origin.
And here all our thinking, feeling, and willing are done.

DIVISION OF LABOR