THE CEREBRAL CIRCULATION 953 



In some small birds the ratio is as high as I : 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. 



The Cerebral Circulation. The arrangement of the cerebral blood- 

 vessels has certain peculiarities which it is of importance 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 vertebrals. 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 anastomosis 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 area of distribution without sending 

 communicating branches to make junction with its neighbours. The 

 consequence of these two anatomical facts is: (i) that interference with 

 the blood-supply of the brain between the heart and the circle of Willis 

 does not readily produce symptoms of cerebral anaemia; (2) that the 

 blocking of any of the arteries which arise from the circle or any of their 

 branches leads to destruction of the area supplied by it. Nearly all 

 dogs recover after ligation in one operation of both carotids and both 

 vertebral arteries. In monkeys both carotids may, as a rule, be safely 

 tied, and one carotid in man. If, in addition to the two carotids, one 

 vertebral be ligated at the same time in the monkey, sopor results, and 

 this is generally followed by extensor rigidity, coma, and death in 

 twenty-four hours. In one case a monkey survived this triple ligation, 

 but became demented. The motor paralysis and rigidity were much 

 greater than in the dog. In the condition of partial anaemia the cortex 

 is more excitable than normal, but the excitability disappears at once 

 when the anaemia is rendered complete (Hill) . 



The basal ganglia are fed by twigs from the circle ol Willis and the 

 beginning of the posterior, middle, and anterior cerebral arteries. Of 

 these the most important are the lenticulo-striate and lenticulo-optic 

 branches of the middle cerebral, which are given off near its origin, and 

 run through the lenticular nucleus into the internal capsule, and thence 

 to the caudate nucleus and optic thalamus respectively. The chief part 

 of the blood from the circle of Willis is carried by the three great cerebral 

 arteries over the cortex of the brain. The white matter, with the 

 exception of that in the immediate neighbourhood of the basal ganglia, 

 is nourished by straight arteries which penetrate the cortex. The 

 middle cerebral supplies the whole of the parietal as well as that portion 

 of the frontal lobe which lies immediately in front of the fissure of 

 Rolando and the upper part of the temporal lobe. The rest of the 



