CHAP. X.] CIRCULATION IN THE BRAIN. 293 



in the fissures of Sylvius, and supply the outer convolutions of the 

 anterior lobes, and the principal portion of the middle lobes. 

 At the inner extremity of each fissure of Sylvius numerous small 

 branches of these arteries penetrate, to be distributed to the corpus 

 striatum. The choroid arteries which supply the choroid plexus 

 sometimes arise from these arteries, but also occasionally come from 

 the carotid itself. The posterior communicating artery is an ana- 

 stomotic vessel, which passes backward along the inner margin 

 of the middle lobe on the base of the brain, and communicates 

 with the posterior cerebral artery, a branch of the basilar. 



The vertebral arteries having passed through the canals in the 

 transverse processes of the cervical vertebrae, enter the cranium 

 through the occipital foramen towards its anterior part. In their 

 ascent they incline towards each other in front of the medulla 

 oblongata, and at the posterior margin of the pons they coalesce 

 to form a single vessel, the basilar, which extends the whole length 

 of the pons. 



The vertebral arteries furnish the anterior and posterior spinal 

 arteries, and the inferior cerebellar arteries. These last vessels 

 arise from the vertebrals very near their coalescence, and pass 

 round the medulla oblongata to reach the inferior surface of the 

 cerebellum, to which they are principally distributed. 



The basilar artery sends numerous small vessels to penetrate 

 the pons, and at its anterior extremity divides into four arteries, 

 two on each side : these are, the two superior cerebellar , and the 

 two posterior cerebral arteries. 



The superior cerebellar arteries pass backwards round the crus 

 cerebri, parallel to the fourth nerve, and divide into numerous 

 branches on the upper surface of the cerebellum, some of which 

 anastomose with branches of the inferior cerebellar artery over the 

 posterior margin of the cerebellum. Some branches of these ar- 

 teries are distributed to the velum interpositum. 



The posterior cerebral arteries are the largest branches of the 

 basilar. They diverge and pass upwards and backwards round 

 the crus cerebri, and reach the inferior surface of the posterior lobe, 

 anastomosing in the median fissure with ramifications of the anterior 

 cerebral, and on the outside with branches of the middle cerebral 

 arteries. Numerous small vessels pass from this artery at its 

 origin, and penetrate the interpeduncular space, and one or two 

 are distributed to the velum. Shortly after its origin, the artery 

 receives the posterior communicating branch from the carotid. 



A remarkable freedom of anastomosis exists between the arteries 



