500 COMPARATIVE PHYSIOLOGY. 



and the monkey, we have thought it well to bring the subject 

 before the student of comparative medicine, since it can not be 

 doubted that future research will put the physiology of the 

 brains of the domesticated animals in a new light, in doing which 

 guidance will naturally be sought from what has been already 

 done, more especially in the case of the human subject and his 

 nearest allies. Some would maintain that in the case of the 

 dog, motor and sensory localization has been established ; that 

 in this animal there is a naotor area in the region of the crucial 

 sulcus corresponding to that around the fissure of Rolando in 

 man. The subject is, however, far from finally settled even in 

 the case of the dog, the brain of which has been more thoroughly 

 investigated than that of any other of our domestic animals. 

 Very little can as yet be said in regard to cortical localization 

 in the horse, ox, etc. It seems highly probable that investiga- 

 tion will show that cortical localization in the primates (man 

 and the monkey tribe) exists in a far higher degree than in 

 any other animals. 



The Circulation in the Brain. — The brain, being inclosed 

 within an air-tight bony case, its circulation is of necessity 

 peculiar. Since any undue compression of the encephalon may 

 lead to even a fatal stupor, it is clear that there must exist some 

 provision to permit of the excess of arterial blood that is re- 

 quired for unusual activity of the brain. It is to be borne in 

 mind that the fluid within the ventricles is continuous, through 

 the foramen of Magendie in the roof of the fourth ventricle, 

 with that surrounding the spinal cord (spinal cavity) ; so that 

 an increase in the volume of the encephalon in consequence of 

 an afllux of blood might be in some degree compensated by an 

 efflux of the cerebro-spinal fluid. The part played by this ar- 

 rangement has, however, been probably overestimated. But 

 the peculiar venous sinuses do, it is likely, serve to regulate the 

 blood-supply ; being very large, they may answer as temporary 

 overflow receptacles. An inspection of the fontanelles of an 

 infant reveals a beating corresponding with the pulse; and, 

 when a large part of the cranium is removed in an animal, a 

 plethysmograph shows a rise in volume corresponding with 

 the pulse and the respiratory movements, as in the case of the 

 fontanelles. But, besides these, periodic waves of contraction 

 are now known to pass over the cerebral arteries. 



Whether the latter is part of a general wave traversing the 

 whole arterial system is as yet uncertain. Though there is 



