THE CENTRAL NERVOUS SYSTEM 879 



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 excit- 

 ability disappears at once when the anaemia is rendered complete (Hill) . 

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

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

 Of these the most importart 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 imme- 

 diate 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 frontal lobe is 

 supplied by the anterior cerebral, and the occipital lobe, with the 

 lower part of the temporal lobe, by the posterior cerebral. The 

 medulla oblongata, cerebellum, and pons are fed from the ver- 

 tebrals and the basilar artery before the circle of Willis has been 

 formed. 



Resuscitation of the Central Nervous System after Total 

 Anaemia. Complete temporary anaemia of the brain and upper 

 cervical cord can be produced in most cats by passing temporary 

 ligatures around the innominate artery and left subclavian 

 proximal to the origin of the vertebral artery. Artificial respira- 



