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 {125} 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. [Illustration: FIG. 18.--AN ARTERY FROM THE CEREBRAL CORTEX.--One can see numerous fine fibers passing over to the brain substance (Obersteiner).] 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 understand.