THE ARTERIES OF THE BRAIN 619 



the external striate, ascends through the outer segment of the lenticular nucleus, 

 and supplies the caudate nucleus and tlv.- thalamus. One artery of this group 

 (also called lenticulostriate arteries) is of larger size than the rest, and is of special 

 importance, as being the artery in the brain most frequently ruptured; it has been 

 termed by Charcot the artery of cerebral hemorrhage. It passes up between the 

 lenticular nucleus and the external capsule, and ultimately ends in the caudate 

 nucleus. 



The inferior external frontal supplies the subfrontal convolution (Broca's convolu- 

 tion) and the outer part of the orbital surface of the frontal lobe. 



The ascending frontal supplies the precentral gyre. 



The ascending parietal is distributed to the postcentral convolution and the 

 lower part of the superior parietal convolution. 



The parietotemporal supplies the supramarginal and angular gyres, the super- 

 temporal, and part of the meditemporal gyre. 



The temporal branches, two or three in number, are distributed to the outer 

 surface of the temporal lobe. 



The posterior communicating artery (a postcommunicans, a. communicans 

 posterior) runs backward from the internal carotid, and anastomoses with the 

 posterior cerebral, a 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. 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 

 perforated substance and supply the internal surfaces of the thalami and the walls 

 of the third ventricle. 



The choroid artery (a. praechoroidea, a. chorioidea) 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, it enters the descending horn of the lateral ventricle through 

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

 campus, fimbria, velum interpositum, and choroid plexus. 



THE ARTERIES OF THE BRAIN. 



Investigations 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 nerve system may be the seat; it is therefore important to 

 consider a little more in detail the manner 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 (circulus arteriosus) (Fig. 454). It is formed in front by the anterior 

 cerebral arteries, branches of the internal carotid, which are connected by the ante- 

 rior communicating; behind by the two posterior cerebrals, branches of the basilar, 

 which are connected on each side to the internal carotid bv the posterior communi- 

 cating (Fig. 449). The parts of the brain included within this arterial circle 

 are the lamina terminalis, the chiasm or commissure of the optic nerves, the tuber 

 cinereum, the corpora albicantia. and the posterior perforated substance. This 

 arrangement of the vessels of the circle is not invariable; according to Windle, it 

 is maintained in little more than half the recorded cases. In the other cases 

 there are different variations. 



The three trunks which together supply each cerebral hemisphere arise from 



