Page:Popular Science Monthly Volume 6.djvu/584

566 is an important matter in sustaining the intercranial flow. If the general blood-pressure in the vessels of the head and neck is high and well maintained, then a brain can work up to a much higher power, just as a steam-engine may be worked up to a higher pressure, and so become actually more powerful.

While we recognize the fact that mental conditions are causally associated with the amount of the blood-supply, indeed to a great extent rest upon it, it must not be supposed that I wish to under-estimate the importance of the condition of the cerebral cells themselves, either as to their inherited peculiarities, or as to the conditions produced by the experience of the individual. Such consideration is, however, without the sphere of the present paper, which deals with cerebral manifestations in relation to ordinary disease, and not with those ailments which belong to the province of the alienist physician. To one form of disorder of the cerebral cells alone may reference be made here, and that is as to the effects of mental over-strain. Brain-tissue can be developed by exercise, and worn out by overwork. When this latter condition has been induced, there exists that irritability which forms part of the early stage of the exhaustion of nerve-matter. In all overtried brains there is much irritability and tendency to manifest what we term temper. This fact we learn in time about the individual, but we are somewhat slow to recognize it in the abstract. It is socially desirable and important that such recognition be more general.

In considering the associations existing between cerebral manifestations and certain conditions of the organism, and the effect exercised by the latter upon the former, it is important to bear in mind that the brain is divided into two vascular areas: 1. The anterior, fed by the internal carotids; and, 2. The posterior, fed by the basilar artery; nor is the amount of inosculation in the intercranial circulation such as would allow vicarious action-to make good a deficiency caused by an interference with the direct supply of either area. These two vascular areas contain brain-cells with different properties and functions. There is much reason to believe that the emotions and systematic sensations lie on the posterior area; and that the intellectual and motor powers, together with general peripheral sensations, lie on the anterior area. In other words, the posterior area is associated with the organic processes of the system; the anterior with the animal life—with the relation of the organism to its surroundings. The vaso-motor nerves of these two areas are differently derived. The nerves of the cerebral arteries spring from the lower ganglion of the great sympathetic nerve in the neck, into which run the fibres ascending from the abdomen; while the carotid arteries derive their vasomotor supply from the middle and upper ganglia. Thus we can see how the emotions sympathize with the organic processes, especially those located in the abdomen, and so can see melancholia in a new