DISSECTION OF THE BRAIN, OR ENCEPHALON. 237 



anterior divisions of the right and left cerebro-spinal arteries, whose 

 posterior divisions nnite in the same manner to form the middle spinal 

 artery. The basilar artery passes forwards in the median groove of the 

 medulla, and crosses the pons, in front of which it bifurcates to form 

 the posterior cerebral arteries. In its course the basilar artery detaches 

 on each side, besides numerous vessels to the medulla and pons, the 

 posterior cerebellar arteries. 



The Posterior Cerebellar Arteries are two in number, a right and left. 

 They are detached at different levels from the basilar, behind the pons ; 

 and they turn round the medulla to reach the cerebellum. 



The Posterior Cerebral Arteries diverge from each other in the inter- 

 peduncular space ; and after being connected together by a short 

 transverse branch of considerable volume, and by numerous smaller 

 reticulate twigs, they are joined by the posterior communicating branch 

 of the internal carotid. Each then turns outwards over the crus 

 cerebri to gain the choroid plexus and the posterior part of the cerebral 

 hemisphere. Behind the point at which the vessels are comiected by 

 the short transverse branch, they give off" the anterior cerebellar arteries. 

 Sometimes, as in Fig. 30, the posterior cerebral artery, instead of turnino- 

 outwards as a single vessel, detaches from its outer side two or three 

 branches which wind round the crus. 



The Anterior Cerebellar Arteries are variable in number and disposi- 

 tion, and may arise as branches of the basilar artery. Generally there 

 are two or three on each side, and they turn backwards and outwards 

 over the crus cerebri to gain the front of the cerebellum. 



The Internal Carotid Artery. This vessel begins above the cricoid 

 cai-tilage of the larynx, as one of the terminal branches of the common 

 carotid. It passes upwards and forwards to the foramen lacerum basis 

 cranii, being sustained in a fold of the guttural pouch, and accompanied 

 by some nervous branches from the superior cervical ganglion of the 

 sympathetic. Piercing the sub-sphenoidal sinus, it passes through the 

 foramen into the cavernous sinus, Avithin which it forms a sigmoid cui-ve. 

 It then leaves the sinus, and gaining the deep face of the dura mater, 

 it divides at the margin of the sella turcica of the sphenoid bone into 

 an anterior and a posterior branch. The latter, termed the posterior 

 communicating artery, is reflected backwards to join the posterior cerebral 

 artery. The anterior branch passes forwards, and at the outer side of 

 the optic commissure divides into the middle and anterior cerebral 

 arteries. 



The Middle Cerebral Artery passes outwards across the hemisphere, in 

 the fissure of Sylvius. 



The Anterior Cerebral Artery unites in the mesial plane, above the 

 optic commissure, with the corresponding vessel of the opposite side. 

 The single vessel thus formed receives the meningeal branch of the 



