THE BLOOD-VESSELS OF THE BRAIN. 



573 



volution. The parieto-sphenoidal supplies the superior temporo-sphenoidal con- 

 volution and the angular gyms. 



The posterior communicating artery arises from the back part of the internal 

 carotid, runs directly backward, and anastomoses with the posterior cerebral, a 



FISSURE OF 

 ROLANDO. 



Perforating 



Branches. 



Middle Cerebral 

 Artery. 



FIG. 356. The distribution of the middle cerebral artery. (After Charcot.) 



branch of the basilar. This artery varies considerably in size, being sometimes 

 small, and occasionally so large that the posterior cerebral may be considered as 

 arising from the internal carotid rather than from the basilar. It is frequently 

 larger on one side than on the other side. From the posterior half of this vessel 

 are given off a number of small branches, the postero-median ganglionic branches, 

 which, with similar vessels from the posterior cerebral, pierce the posterior perfo- 

 rated space and supply the internal surfaces of the optic thalami and the walls of 

 the third ventricle. 



The anterior choroid is a small but constant branch which arises from the 

 back part of the internal carotid, near the posterior communicating artery. 

 Passing backward and outward, it enters the descending horn of the lateral ven- 

 tricle beneath the edge of the middle lobe of the brain. It is distributed to the 

 hippocampus major, corpus fimbriatum, velum interpositum, and choroid plexus. 



The Blood-vessels of the Brain. 



Recent investigations have tended to show that the mode of distribution of 

 the vessels of the brain has an important bearing upon a considerable number of 

 the anatomical lesions of which this part of the nervous system may be the seat ; 

 it therefore becomes important to consider a little more in detail the way in which 

 the cerebral vessels are distributed. 



The cerebral arteries are derived from the internal carotid and the vertebral, 

 which at the base of the brain form a remarkable anastomosis known as the circle 

 of Willis. It is formed in front by the anterior cerebral arteries, branches of the 

 internal carotid, which are connected together by the anterior communicating; 

 behind by the two posterior cerebrals, branches of the basilar which are connected 

 on each side with the internal carotid by the posterior communicating (Fig. 354, 

 p. 573). The parts of the brain included within this arterial circle are the lamina 

 cinerea, the commissure of the optic nerves, the infundibulum, the tuber cinereum, 

 the corpora albicantia, and the posterior perforated space. 



From the circle of Willis arise the three trunks which together supply each 

 cerebral hemisphere. From its anterior part proceed the two anterior cerebrals, 

 from its antero-lateral part the middle cerebral, and from its posterior part the 



