Pursuing his researches in other directions, Professor Mosso has succeeded in demonstrating that, when the brain acts, not only does the arm receive less blood, but the brain actually does receive more. This result was, of course, to be anticipated; and in making this further observation Mosso is not the first, for other physiologists have published researches upon this point, only less accurate and complete than those of the distinguished Italian investigator.
Opportunities for observing the circulation of the human brain are very rare, and occur only in consequence of violent accidents or insidious diseases, causing a loss of a piece of the bony skull sufficient to leave the soft brain exposed. Through such an opening it can be observed that the soft brain is smaller during sleep than during waking, because during the former state it draws back from the opening, and during the latter it may swell so much as even to protrude through the opening. That these changes are not abnormal results due to the diseased condition, is shown by the experiments which have been made upon healthy dogs by artificially removing a small piece of the skull, which can be done without causing any serious injury. In animals thus operated upon, the same changes of volume can be seen to occur in the brain, and closer observation shows that the variations are due to contraction or expansion of the blood-vessels.
These investigations demonstrate that one of the physiological conditions of increased mental action is an increased supply of blood, which is produced principally by a dilatation of the cerebral blood-vessels, accompanied by a contraction of the blood-vessels of other parts of the body. The measurable volume of the arm is thus partly a signal of the condition of the mind we can not measure, as affirmed in the early part of this article.
In connection with the new tendencies of psychology and physiology, such investigations as we have just described acquire a peculiar significance. The progress of knowledge has so enlarged the domains of both psychology and physiology, that they now overlap. The fields of investigation held in common form the bourn of "physiological psychology," as it is termed by the Germans, who are ever ready with a new name. Now, the mind derives its material through the senses. The sensations arise from physical causes. The final results of mental performances are various actions of the body, physical events such as motion and speech. Physics, therefore, are the alpha and omega of our mental history. Concerning what occurs between the physical cause of sensation and the physical result of mental action, two extreme opinions stand opposed. On the one hand, mind is defined as a succession of purely physical phenomena; on the other, as a supernatural and immortal power.
Hitherto psychologists have usually studied very little besides what we might call the natural history of the mind. Just as the ornithologist may study the habits of a bird, its mode of hopping, flying, feed-
