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Even more important, perhaps, than any other of the functions
attributed to the neuroglia cells, is the role they may play in
enabling the individual to concentrate attention on a particular
subject, or at least to use a particular portion of his brain,
by bringing about a more active circulation in that portion than in
any other, Ramon y Cajal attributes this power to the perivascular
neuroglia cells. Every capillary in the brain has thousands of these
little pseudopod prolongations. When the cells in a particular region
contract, the blood vessels of the part are pulled wide open and a
larger supply of blood flows more freely, stimulating the nerve cells
by which it passes and supplying them with nutrition for the
expenditure of energy that they may have to make. This is the physical
process that underlies attention. When too much, that is, too
long-continued attention is paid to any subject, without diversion of
mind, the capillaries may easily acquire the habit of being open, and
cells the custom of contraction, so that relaxation does not readily
take place. Something of this kind is the most important element in
the etiology of many functional nervous disorders.

Ease and Pleasure in Mental Operations.--On the other hand this same
set of ideas explains many things otherwise difficult of
understanding. For instance, we all know that habit enables us to
apply ourselves to a particular subject with ever growing ease. What
was extremely difficult for us at the beginning, may after a time
become comparatively easy, and later even positively pleasant. Study,
that is application of mind, is, at the beginning, for most people,
not agreeable. If persisted in, it almost inevitably becomes a
pleasure. Hard exercise of any kind is, at the beginning, sure to
require great energy of purpose, and requires some subsidiary motive
of approbation or reward to make us persist in it. But what was a
distinct labor at the beginning becomes pleasant after a while. This
may be applied to the neuroglia cells apparently as well as to the
muscle fibers. On this theory, the reason for the gradual acquirement
of an intense pleasure in the intellectual life becomes easy to

Dangers of Over-attention.--The danger of concentration of mind on
one's self, quite as much as on any other subject, becomes clearer
when this theory is accepted as explaining the physical basis of the
mental operations involved in attention. If people allow thoughts of
themselves and of their physical processes constantly to occupy their
minds, gradually that portion of the brain ruling over these becomes
over-fatigued and fails to respond to the calls for relaxation.
Insomnia may develop readily as a consequence of continued solicitude
and prove to be, as the worst forms of insomnia so often are, quite
unamenable to direct drug treatment, because, even during the enforced
sleep that comes from drugs, dreams with regard to self and the
supposed ills may still occupy the overworked portion of the brain.
Nervous people are, most occupied with those parts of the brain which
have something to do with the omission and transmission of trophic
influence to particular parts of the body. As a consequence of the
persistent hyperemia, too many trophic impulses are sent down. These
cause an exaggeration of physiological function, in the stomach, the
heart, or some other important organ. Hence these organs may become

For all these reasons, this theory of attention, of the great Spanish
investigator, deserves to be well known by those who hope to
treat neurotic affections, especially functional diseases of the
brain, and therefore I prefer once more to give it in his own words.
[Footnote 16]

[Footnote 16: International Clinics, Vol II, Series 11.]

Ramon y Cajal's Theory of Attention.--Under usual conditions, the
motor apparatus of the gray matter suffices for the explanation or
the varied course of association of ideas and of the reaction
produced by voluntary motion. But as soon as attention is
concentrated upon an idea, or a small number of associated ideas,
there enters into the problem, besides the active retraction of the
neuroglia of the corresponding part of the brain, a new factor--the
active congestion of the capillaries of the over-excited region. As
a consequence of this, the energy of emotion reaches a maximum. The
heat and metabolism of the hyperemic parts is increased, which, of
course, makes these parts capable of more work.

This congestion of various parts of the brain has been experimentally
observed by a number of physiologists. It can be best explained
by considering that the will has an influence upon the nerves which
produce a dilatation of the blood-vessels in different parts of the
cerebral cortex. The process of attention, however, by which
intellectual activity is concentrated upon a limited number of ideas,
seems to be but very little under the control of the sympathetic nerve

As a matter of fact, the capillaries of the brain are wanting in
nerves and smooth muscle fibers. Hence they are not under the control
of the sympathetic system. Only the relatively large arteries of the
pia mater, which possesses a tunica muscularis are under a certain
limited control of the sympathetic, which is able to produce in them
an incomplete and not very well limited congestion. One of the
difficulties of the problem of the activity of the sympathetic is best
realized when we recall that vasomotor activity is usually
involuntary. The process of attention, however, is entirely conscious
and voluntary.

In the hypothesis that we have given, most of the difficulties
disappear. Under the influence of the will, the pseudopod branches of
the neuroglia cells, which end in the walls of the capillaries,
contract. As the result of this, the bloodvessels, all of which are
surrounded by lymph spaces, dilate, and this dilatation may proceed to
such an extent that the vessels occupy the whole of the lymph spaces.
Thus we can easily understand how the very limited congestions which
are necessary for the concentration of thought upon a single idea may
be brought about.

The perivascular lymph spaces which exist in the brain seem to be for
the purpose of making these limited hyperemias easier. At the same
time they serve a very useful purpose in preventing pressure or
concussion, such as might be caused upon the neighboring nerve cells
by too great dilatation of the blood vessels of a part.

It is needless to add that we do not consider the hypothesis that we
have advanced to be absolutely without objection. On the contrary we
believe that, owing to the difficulty of the problem and our, as yet,
extremely slight knowledge of the anatomy and physiology of the nerve
protoplasm, any theory as to the special mechanism of psychic
processes is sure to be faulty. Rational hypotheses, however, which
are supported by well-known facts, are not only justified, but are
often fruitful of suggestive ideas. A scientific hypothesis often
gives a new direction, suggests an untried method of observation, or
hints at new ways of experiment, and, though it may not lead directly
to truth, always brings us closer to methods of investigation and of
criticism that are invaluable. Even though our further investigations
should not confirm our hypothesis, the result will not be less
positive. Negative conclusions lessen the number of possible
hypotheses and therefore diminish the possibility of error in future

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