SPECIAL ANATOMY. 161 



oculorum, arises. Between these, anteriorly is the tuhcrr 

 cinereum, which is continued to the pituitary body as the 

 infiindibulum, and through which a canal runs. Imme- 

 diately behind this is the corims albicans, formed by the 

 anterior crura of the fornix, and more ^posteriorly, between 

 the crura cerebri in the human subject, is locus jjerforatus' 

 jposticus, ivJiicli in the horse is represented merely by a groove^ 

 The hemispheres of the cerebrum of man are divided into 

 anterior, posterior, and middle lobes. At the lateral part 

 of the crura cerebri of the horse is a smooth, round mass, 

 mastoid lobule (analogous to the middle lobe of man). 

 From the optic decussation in front of this the fissure of 

 Sylvius runs outwards, tending to divide the hemisphere 

 into an anterior and a posterior lobe. Before running over 

 the under surface of pons Varolii the basilar artery gives 

 off the anterior, posterior, and middle cerebellal arteries, after 

 doing so it terminates in receiving the two posterior com- 

 municating arteries. These give off the posterior cerebral 

 arteries (which run upwards winding round the crura 

 cerebri) and are given off with the anterior communicating 

 and middle cerebral by the internal carotid after it has 

 pierced the dura mater. The anterior communicating artery 

 runs inwards and blends with its fellow in front of the 

 optic decussation. The middle cerebral runs outwards 

 through the fissure of Sylvius. From the anterior commu- 

 nicating or from the middle cerebral the ophthalmic arteries 

 pass through the foramina lacera orbitale to the orbit, and 

 from the centre of the anterior communicating the 

 anterior cerebral artery (double in the human subject) 

 passes around the anterior extremity of the corpus callosum 

 into the superior longitudinal fissure, where it terminates 

 in forming the arteria corporis callosi. Thus we have the 

 circle of Willis at the base of the brain formed anteriorly by 

 the anterior communicating, laterally and posteriorly by the 

 posterior communicating arteries ; it encloses the optic decus- 

 sation, tuber cinereum, infundibulum, corpus albicans, locu& 

 perforatus, and part of the crura cerebri, and surrounds the 

 pituitary body. We have always found the origin of the 

 third pair of nerves externally placed to it. Posteriorly at 

 the base of the brain we see the pons Varolii running trans- 

 versely, and a fifth nerve arising at either end of it. Fromj 

 the under surface of the medulla oblongata, just behind 

 the pons, on either side of the inferior longitudinal fissure, 



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