HEAD AND NECK OF THE HORSE 175 



origin. There may be only one on each side of the brain, but very 

 frequently there are two or three, one or more of which may arise from 

 the posterior cerebral artery. Bending round the lateral border of the 

 mid-brain and following the brachium pontis, they terminate mainly on 

 the anterior surface of the cerebellum. (4) The posterior cerebral 

 arteries (aa. cerebri posteriores) leave the termination of the basilar at 

 an acute angle, and each is soon joined by a posterior communicating 

 branch of internal carotid artery. The vessel then disappears from the 

 present dissection by passing into the narrow chink between the 

 cerebral hemisphere and the cerebral peduncle. The posterior cerebral 

 arteries are chiefly concerned in supplying the posterior part of the 

 cerebral hemispheres. 



A. carotis interna. — After traversing the cavernous venous sinus 

 and piercing the dura mater, the internal carotid artery terminates im- 

 mediately lateral to the optic chiasma by dividing into middle and 

 anterior cerebral arteries. A collateral branch, the posterior com- 

 municating artery, leaves the internal carotid as soon as it has 

 penetrated the dura mater. 



The posterior communicating artery (a. communicans posterior) 

 runs backwards on the cerebral peduncle and joins the posterior 

 cerebral. A small and inconstant deep cerebral artery (a. cerebri pro- 

 funda) bends round the border of the peduncle and ends in the mid-brain. 



The middle cerebral artery (a. cerebri media) gains the lateral cere- 

 bral (Sylvian) fissure by traversing the depression anterior to the piriform 

 lobe, and is distributed over almost the whole of the lateral surface of 

 the cerebral hemisphere. 



A small choroidal arteo'y (a. chorioidea) leaves either the middle 

 cerebral close to its commencement or the internal carotid near its 

 termination, and follows the optic tract to end in the choroid plexus of 

 the lateral ventricle. 



The anterior cerebral artery (a. cerebri anterior), converging upon 

 the median plane, passes dorsal to the optic nerve and unites with its 

 fellow of the opposite side. The single vessel thus produced enters the 

 longitudinal fissure and bends round the genu of the corpus callosum. 

 It then divides into right and left branches and ramifies over the medial 

 surface of the cerebral hemisphere. A small anterior meningeal 

 artery (a. meningea anterior) arises from the anterior cerebral and 

 supplies blood to the anterior part of the dura mater, anastomosing 

 with branches of the ethmoidal artery to form a network in the 

 ethmoidal fossa. It is often also possible to demonstrate another small 

 branch of the anterior cerebral artery, which follows the optic nerve and 

 anastomoses with a branch of the ophthalmic artery. 



