1342 A MANUAL OF ANATOMY 



Arteries at the Base of the Encephalon. — ^The arteries which 

 supply the brain are the two vertebral and the two internal carotid 

 arteries. 



The vertebral arteries incline inwards as they ascend on the 

 ventral aspect of the medulla oblongata, and at the lower border 

 of the pons Varolii they unite to form the basilar artery. The 

 branches of each vertebral artery to be noted are as follows : 

 (i) the posterior spinal branch, which arises from the main vessel 

 immediately after it has pierced the dura mater, and descends upon 

 the side of the medulla oblongata to the spinal cord ; (2) the anterior 

 spinal branch, which arises higher up than the preceding, and 

 passes downwards and inwards on the ventral aspect of the medulla 

 oblongata to unite with its fellow and form the anterior spinal 

 artery ; and (3) the posterior inferior cerebellar branch, of large size, 

 which arises from the main vessel near the pons, and passes back- 

 wards round the medulla oblongata to enter the vallecula of the 

 cerebellum. 



The basilar artery extends from the lower border of the pons 

 Varolii to the upper border, occupying the basilar groove on its 

 ventral surface. It is formed by the union of the two vertebral 

 arteries, and terminates by dividing into the two posterior cerebral 

 arteries. The branches of the basilar artery to be noted on either 

 side are as follows : (i) the transverse arteries of the pons ; (2) the 

 internal auditory artery, which accompanies the auditory nerve 

 through the meatus auditorius internus ; (3) the anterior inferior 

 cerebellar artery, which arises from the basilar about its centre, 

 and passes backwards to the inferior surface of the cerebellar 

 hemisphere ; (4) the superior cerebellar artery, which arises from the 

 basilar near its termination, and passes outwards close to the upper 

 border of the pons, and then round the outer side of the crus cerebri 

 to the superior surface of the cerebellar hemisphere ; and (5) the 

 posterior cerebral artery, which arises from the termination of the 

 basilar, and passes outwards, parallel to the superior cerebellar 

 artery, and then round the outer side of the cms cerebri to the 

 inferior surface of the occipital lobe. The posterior cerebral and 

 superior cerebellar arteries are separated from each other by the 

 third and fourth cranial nerves, the latter being the external of 

 the two. The branches of the posterior cerebral artery to be noted, 

 amongst others, are as follows : (i) postero-mesial, which pass to 

 the locus perforatus posticus ; (2) postero-lateral, which pass round 

 the outer side of the crus cerebri ; and (3) posterior choroidal, which 

 pass to the upper part of the choroidal fissure. 



The internal carotid artery of each side appears at the vallecula 

 Sylvii, and there divides into the anterior and middle cerebral 

 arteries. Near its termination it gives off the posterior communicat- 

 ing artery, which passes backwards to join the posterior cerebral 

 artery. It also gives off the anterior choroidal artery, which 

 passes backwards and outwards between the crus cerebri and the 

 uncinate gyrus to the lower and anterior part of the choroidal fissure. 



