908 THE VASCULAB SYSTEM. 



oculo-motor nerve of the same side, and, after turning round the lateral side of the 

 pedunculus cerebri, below the trochlear nerve, it reaches the 'upper surface of the 

 cerebellum, where it divides into a medial and a lateral branch. The medial branch 

 supplies the upper part of the vermis, and the anterior medullary velum. The lateral 

 branch is distributed over the upper surface of the lateral lobe; it anastomoses with the 

 inferior cerebellar arteries. 



(5) Arterise Cerebri Posteriores. The posterior cerebral arteries (Figs. 762 and 

 764) are the two terminal branches of the basilar. They run backwards and 

 upwards, between the peduncles of the cerebrum and the uncinate gyri and parallel 

 to the superior cerebellar arteries, from which they are separated by the oculo- 

 motor and trochlear nerves. Each posterior cerebral artery is connected with the 

 internal carotid by the posterior communicating artery; it gives branches to the 

 inferior surface of the cerebrum, and is continued backwards, beneath the splenium 

 of the corpus callosum, to the calcarine fissure, where it divides into calcarine and 

 parieto-occipital branches, which pass to the lateral surface of the occipital lobe. 

 It supplies the medial and tentorial surfaces of the occipital lobe and the posterior 

 part of its lateral surface. 



Branches. (A) Central or basal. This group includes (a 1 ) A postero-medial Bet 

 of small vessels which pass, on the medial side of the corresponding cerebral peduncle, 

 to the posterior perforated substance. They supply the peduncle, the posterior part of 

 the thalamus, the corpora mamillaria, and the walls of the third ventricle. 



(a 2 ) A postero-lateral set of small vessels, which pass round the lateral side of the 

 peduncle. They supply the corpora quadrigemina, the brachia, the pineal body, the 

 peduncle, the posterior part of the thalamus, and the corpora geniculata. 



(a 3 ) A posterior chorioidal set of small branches which pass through the upper part of 

 the chorioidal fissure ; they enter the posterior part of the tela chorioidea of the third 

 ventricle, and end in the chorioid plexus, in the body of the lateral ventricle, and the 

 upper part of its inferior cornu. They also supply the adjacent parts of the fornix. 



(B) Cortical. (6 1 ) The anterior temporal, frequently a single branch of variable size, 

 but not uncommonly replaced by several small branches. It supplies the anterior parts of 

 the uncus, the hippocampal gyrus, and the fusiform gyrus. 



(6 2 ) The posterior temporal is a larger branch than the anterior. It supplies the 

 posterior part of the hippocampal gyrus, part of the fusiform gyrus, and the lingual gyrus. 



(6 3 ) The calcarine branch is the continuation of the posterior cerebral artery along 

 the calcarine fissure, it is especially associated with the supply of the visual area of the 

 cortex of the brain. It supplies the cuneus, the lingual gyrus, and the posterior part of 

 the lateral surface of the occipital lobe. 



(6 4 ) The parieto-occipital branch, smaller than the calcarine, passes along the 

 corresponding fissure to the cuneus and praecuneus. 



Circulus Arteriosus (Willis) (Fig. 764). The cerebral arteries of opposite sides 

 are intimately connected together at the base of the brain by anastomosing channels. 

 Thus, the two anterior cerebral arteries are connected with one another by the 

 anterior communicating artery, whilst the two posterior cerebrals are in continuity 

 through the basilar artery from which they arise. There is also a free anastomosis 

 on each side between the carotid system of cerebral arteries and the vertebral 

 system by means of the posterior communicating arteries, which connect the 

 internal carotid trunks and posterior cerebral arteries. 



The vessels referred to form the so-called circulus arteriosus (O.T. circle of 

 Willis) which is situated at the base of the brain, in the interpeduncular and 

 chiasmatic subarachnoid cisterns. It encloses the following structures : the 

 posterior perforated substance, the corpora mamillaria, the tuber cinereum, the 

 infundibulum, and the optic chiasma. The "circle" is irregularly polygonal in 

 outline, and is formed posteriorly by the termination of the basilar and by the 

 two posterior cerebral arteries, postero-laterally by the posterior communicating 

 arteries and the internal carotids, antero-laterally by the anterior cerebral arteries, 

 and in front by the anterior communicating artery. 



It is stated that this free anastomosis equalises the flow of blood to the various 

 parts of the cerebrum, and provides for the continuation of a regular blood-supply 

 if one or more of the main trunks should be obstructed. 



