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