76o HUMAN ANATOMY. 



nerves, and, curving upward in the tentorial fissure almost parallel with' the trochlear nerves, 

 are distributed to the upper surface of the cerebellum and anastomose with the inferior cere- 

 bellar arteries. 



{^) The posterior cerebral arteries (aa. cerebri posteriores) (Fig. 702) are the terminal 

 branches of the basilar. From its origin at the anterior border of the pons each artery passes 

 outward and slightly forward, curving around the crus cerebri, immediately in front of the root 

 of the oculomotor nerve, which separates it from the superior cerebellar artery. It then 

 passes upon the inferior surface of the cerebral hemisphere, where it breaks up into cortical 

 branches which ramify over the surface of the temporal and occipital lobes, anastomosing with 

 one another and with the branches of the anteriof and middle cerebrals. The cortical branches 

 (Fig. 700) include the antetior temporal, which supplies the anterior parts of the uncinate and 

 uccipito-temporal convolutions; iho. posterior temporal, distributed to the posterior part of the 

 uncinate and the occipito-temporal convolutions and the adjoining gyrus lingualis ; the calca- 

 } ine, the continuation of the posterior cerebral along the calcarine fissure, which passes to the 

 cuneus and the g>'rus lingualis, and winds to the outer surface ; and \h*t parieto-occipital, which 

 follows the parieto-occipital fissure to the cuneus and the quadrate lobe. 



Immediately at their origin the posterior cerebrals give rise to a number of small central 

 branches {postero-mesial and postcro-latcral ga7iglionic) which dip down into .the substance 

 of the posterior perforated space to supply the optic thalamus and the adjacent parts of the 

 brain-stem, and somewhat more laterally each gives off a posterior choroidal branch, which 

 passes forward in the transverse fissure to the choroid ple.xus of the third ventricle. Near 

 where it passes in front of the oculo-motor nerve, each posterior cerebral receives the posterior 

 communicating artery which passes back to it from the internal carotid, and more laterally it 

 gives off some small branches which are distributed to the corpora quadrigemina and the 

 posterior part of the optic thalamus. 



Variations. — The vertebral artery may arise from a trunk common to it and one of the 

 other branches of the subclavian, and sometimes it arises directly from the arch of the aorta or, 

 on the right side, from the innominate artery or the common carotid. It may traverse a foramen 

 in the transverse process of the seventh cervical vertebra, or the lowest vertebrarterial foramen 

 through which it passes may be the fifth, fourth, third, or even the second. Very rarely the two 

 vertebrals fail to unite to form a single median basilar, that artery being thus represented by two 

 longitudinal trunks united by transverse anastomoses. Occasionally the basilar divides into two 

 longitudinal stems which reunite farther forward, and its formation by the fusion of two parallel 

 vessels is frequently indicated by the presence in its interior of a more or less perfect median 

 sagittal partition. 



The vertebral may give origin to an inferior thyroid artery or to the deep cervical, and oc- 

 casionally, in its upper part, to a branch which anastomoses with the occipital. One of the pos- 

 terior inferior cerebellar arteries may be wanting, as is also not infrequently the case with one 

 of the anterior inferior cerebellars, or these latter vessels may arise from the posterior cerebral. 

 Occasionally the proximal portion of one or other of the posterior cerebral arteries is reduced to 

 a mere thread, the blood reaching the terminal portions of the vessel from the internal carotid, 

 through the posterior communicating artery. 



The Circle of Willis. — The circle or, as it is more properly called, the 

 polygon of Willis (circulus arteriosus) is a continuous anastomosis at the base of the 

 brain (Fig. 702) between branches of the internal carotids and subclavians (verte- 

 brals). It surrounds the posterior perforated space and the floor of the thalamen- 

 cephalon. Posteriorly it is formed by the proximal portions of the posterior cerebral 

 arteries, at the sides by the posterior communicating and internal carotid arteries 

 behind, and by the proximal portions of the anterior cerebrals in front, and it is 

 completed anteriorly by the anterior communicating artery which unites the two 

 anterior cerebrals. 



By means of these connections free communication is established at the base of 

 the brain between the two internal carotids and also between these vessels and the 

 vertebrals. It may be noted that a further communication between these sets of 

 vessels exists upon the lateral surfaces of the cerebral hemispheres where branches 

 of the posterior cerebral arteries anastomose with branches of both the middle and 

 anterior cerebrals. 



In marked contrast to this abundant anastomosis of the larger vessels upon the 

 surface of the cerebrum is the lack of direct communication between the small vessels 

 which penetrate its .substance. These are all terminal or end-arteries, — that is, 

 vessels which have no communication with others except through the general capil- 

 lary net-work, which offers but little opportunity for the establishment of an efificient 

 collateral circulation in the case of occlusion of one of the arteries. 



