512 



THE BLOOD-VASCULAR SYSTEM. 



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, 

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

 rated space and supply the internal surfaces of the optic thalami and the Avails 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 between the temporal lobe and the crus cerebri, it 

 enters the descending horn of the lateral ventricle through the transverse fissure 

 and ends in the choroid plexus. 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 Avhich are connected 

 on each side with the internal carotid by the posterior communicating (Fig. 291, 

 p. 507). The parts of the brain included Avitliin 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. 



or cerebral artery. 



InternaTcavotid artery. 



Middle cerebral artery. 

 I 



/Posterior cerebral artery. 



FIG. 296. Diagram of the arterial circulation at the base of the brain. (After Charcot.) I. Antero-median 

 group of ganglionic branches. II. Postero-median group. III. Right and left antero-lateral group. IV. Right 

 and left postero-lateral group. The dotted line shows t'.ie limit of the ganglionic circle. 



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 cerebrals, and from its posterior part the 



