766 



A MANUAL OF PHYSIOLOGY 



Age. 



8 years. 

 10 

 ii 



12 



13 

 14 

 15 



man at different ages, show that, although we might give ' the infant 

 phenomenon ' an anatomical basis, we should greatly overrate the 

 intellectual acuteness of the average baby if we were to measure it 

 by the ratio of brain to body-weight alone. 



Brain-weight. 

 1045 g rm - 



13*5 

 1168 

 1286 



I 55 

 1336 

 1414 



(Bischoff.) 



Women. 



1239 grm. 



1219 



1129 ,. 



898 



(Huschke.) 



In some small birds the ratio is as high as 1:12, in large birds as 

 low as i : 1,200 ; in certain fishes a gramme of brain has to serve for 

 over 5 kilos of body. As a rule, especially within a given species, 

 the brain is proportionally of greater size in small than in large 

 animals. It is to be supposed that quality as well as quantity of 

 brain substance is a potent factor in determining the degree of 

 mental capacity. Such investigations as those of Berkley, who states 

 that he has made out in a case of dementia in a man addicted to 

 alcohol and opium, changes in the dendrites of pyramidal cells in 

 the cerebral cortex (diminution in their diameter, and in the number 

 of gemmules and branches), are of interest in this connection. 



The Cerebral Circulation. The arrangement of the cerebral 

 bloodvessels has certain peculiarities which it is of great import- 

 ance to remember in connection with the study of the diseases 

 of the brain, many of which are caused by lesions in the 

 vascular system haemorrhage or embolism. Four great arterial 

 trunks carry blood to the brain, two internal carotids and two verte- 

 brais (Plate V., 4). The vertebrals unite at the base of the skull 

 to form the single mesial basilar artery, which, running forward in a 

 groove in the occipital bone, splits into the two posterior cerebral 

 arteries. Each carotid, passing in through the carotid foramen, 

 divides into a middle and an anterior cerebral artery ; the latter 

 runs forward in the great longitudinal fissure, the former lies in 

 the fissure of Sylvius. A communicating branch joins the middle 

 and posterior cerebrals on each side, and a short loop connects the 

 two anterior cerebrals in front. In this way a hexagon is formed at 

 the base of the brain, the so-called circle of Willis. While the anas- 

 tomosis between the large arteries is thus very free, the opposite is 

 true of their branches. All the arteries in the substance of the brain 

 and cord are ' end-arteries '; that is to say, each terminates within its 



