3 io THE POPULAR SCIENCE MONTHLY. 



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 fur- 

 ther 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 insid- 

 ious 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 ob- 

 served 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 ani- 

 mals 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 physi- 

 ology, 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 ex- 

 treme 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 ornitholo- 

 gist may study the habits of a bird, its mode of hopping, flying, feed- 



