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Localization 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.

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 IN THE CORTEX.--Nor does the division of labor in the
nervous system end with this assignment of work. The cortex itself
probably works essentially as a unit, yet it is through a shifting of
tensions from one area to another that it acts, now giving us a
sensation, now directing a movement, and now thinking a thought or
feeling an emotion. Localization of function is the rule here also.
Certain areas of the cortex are devoted chiefly to sensations, others to
motor impulses, and others to higher thought activities, yet in such a
way that all work together in perfect harmony, each reenforcing the
other and making its work significant. Thus the front portion of the
cortex seems to be devoted to the higher thought activities; the region
on both sides of the fissure of Rolando, to motor activities; and the
rear and lower parts to sensory activities; and all are bound together
and made to work together by the association fibers of the brain.

In the case of the higher thought activities, it is not probable that
one section of the frontal lobes of the cortex is set apart for
thinking, one for feeling, and one for willing, etc., but rather that
the whole frontal part of the cortex is concerned in each. In the motor
and sensory areas, however, the case is different; for here a still
further division of labor occurs. For example, in the motor region one
small area seems connected with movements of the head, one with the arm,
one with the leg, one with the face, and another with the organs of
speech; likewise in the sensory region, one area is devoted to vision,
one to hearing, one to taste and smell, and one to touch, etc. We must
bear in mind, however, that these regions are not mapped out as
accurately as are the boundaries of our states--that no part of the
brain is restricted wholly to either sensory or motor nerves, and that
no part works by itself independently of the rest of the brain. We name
a tract from the predominance of nerves which end there, or from the
chief functions which the area performs. The motor localization seems to
be the most perfect. Indeed, experimentation on the brains of monkeys
has been successful in mapping out motor areas so accurately that such
small centers as those connected with the bending of one particular leg
or the flexing of a thumb have been located. Yet each area of the cortex
is so connected with every other area by the millions of association
fibers that the whole brain is capable of working together as a unit,
thus unifying and harmonizing our thoughts, emotions, and acts.





Next: Forms Of Sensory Stimuli

Previous: Gross Structure Of The Nervous System



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