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ANGIOLOGY 



cerebral, a branch of the basilar. It varies in size, being sometimes small, and occa- 

 sionally 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. From its posterior half are given off a number of small branches, 

 the postero-medial ganglionic branches, which, with similar vessels from the posterior 

 cerebral, pierce the posterior perforated substance and supply the medial surface 

 of the thalami and the walls of the third ventricle. 



11. The anterior choroidal (a. chorioidea; choroid artery) is a small but constant 

 branch, which arises from the internal carotid, near the posterior communicating 

 artery. Passing backward and lateralward between the temporal lobe and the 

 cerebral peduncle, it enters the inferior horn of the lateral ventricle through the 

 choroidal fissure and ends in the choroid plexus. It is distributed to the hippo- 

 campus, fimbria, tela chorioidea of the third ventricle, and choroid plexus. 





THE ARTERIES OF THE BRAIN. 



Int. carotid 



Ant. communicating 

 Ant. cerebral 



Post com- 

 municating 



Since the mode of distribution of the vessels of the brain has an important 

 bearing upon a considerable number of the pathological lesions which may occur 



in this part of the nervous system, it is im- 

 portant to consider a little more in detail the 

 manner in which the vessels are distributed. 

 The cerebral arteries are derived from the 

 internal carotid and vertebral, which at the 

 base of the brain form a remarkable anasto- 

 mosis known as the arterial circle of Willis. 

 It is formed in front by the anterior cere- 

 bral arteries, branches of the internal carotid, 

 which are connected together by the anterior 

 communicating; behind by the two posterior 

 cerebral arteries, branches of the basilar, 

 which are connected on either side with the 

 internal carotid by the posterior commu- 

 nicating (Figs. 516, 519). The parts of the 

 brain included within this arterial circle are 

 the lamina terminalis, the optic chiasma, 

 the infundibulum, the tuber cinereum, the 

 corpora mammillaria, and the posterior 

 perforated substance. 



The three trunks which together supply 

 each cerebral hemisphere arise from the 

 arterial circle of Willis. From its anterior 

 part proceed the two anterior cerebrals, 

 from its antero-lateral parts the middle 

 cerebrals, and from its posterior part the t 

 posterior cerebrals. Each of these prin- 

 cipal arteries gives origin to two different 



systems of secondary vessels. One of these is named the ganglionic system, and the 

 vessels belonging to it supply the thalami and corpora striata ; the other is the cortical 

 system, and its vessels ramify in the pia mater and supply the cortex and subjacent 

 brain substance. These two systems do not communicate at any point of their 

 peripheral distribution, but are entirely independent of each other, and there is 

 between the parts supplied by the two systems a borderland of diminished 

 nutritive activity, where, it is said, softening is especially liable to occur in the 

 brains of old people. 



Posterior 

 inferior 

 cerebettar 



FIG. 519. Diagram of the arterial circulation at 

 the base of the brain. A.L. Antero-lateral. A.M. 

 Antero-medial. P.L. Postero-lateral. P.M. Postero- 

 medial ganglionic branches. 



