BASE OF THE BRAIN. 423 



of the pia mater, and the circle of Willis, must be carefully cleared 

 away in order to expose all the parts to be examined. These he will 

 find arranged in the following order from before backwards : 



Longitudinal fissure, 



Olfactory nerves, 



Fissure of Sylvius, 



Substantia perforata, 



Commencement of the transverse fissure, 



Optic commissure, 



Tuber cinereum, 



Infundibulum, 



Corpora albicantia, 



Locus perforatus, 



Crura cerebri, 



Pons Varolii, 



Crura cerebelli, 



Medulla oblongata. 



The Longitudinal fissure is the space separating the two hemi- 

 spheres; it is continued downwards to the base of the brain, and 

 divides the two anterior lobes. In this fissure the anterior cerebral 

 arteries ascend towards the corpus callosum ; and, if the two lobes be 

 slightly drawn asunder, the anterior border (genu) of the corpus cal- 

 losum will be seen descending to the base of the brain. Arrived at 

 the base of the brain, the corpus callosum terminates by a concave 

 border which is prolonged to the commissure of the optic nerves by a 

 thin layer of grey substance, the lamina cinerea. The lamina cinerea 

 is the anterior part of the inferior boundary of the third ventricle. 

 On each side of the lamina cinerea the corpus callosum is continued 

 into the substantia perforata and crura cerebri, and upon the latter 

 forms a narrow medullary band lying externally to, and slightly over- 

 lapping the optic tract, the medulla innominata. 



Upon the under surface of each anterior lobe, on either side of the 

 longitudinal fissure, is the olfactory nerve, with its bulb. 



The Fissure of Sylvius bounds the anterior lobe posteriorly, and 

 separates it from the middle lobe ; it lodges the middle cerebral 

 artery. If this fissure be followed outwards, a small isolated cluster 

 of five or six convolutions will be observed ; these constitute the is- 

 land of Reil. The island of Reil, together with the substantia per- 

 forata, form the base of the corpus striatum. 



The Substantia perforata is a triangular plane of white substance, 

 situated at the inner extremity of the fissure of Sylvius. It is named 

 perforata, from being pierced by a number of openings for small arte- 

 ries, which enter the brain in this situation to supply the grey sub- 

 stance of the corpus striatum. 



Passing backwards on each side beneath the edge of the middle 

 lobe, is the commencement of the great transverse fissure, which ex- 

 tends beneath the hemisphere of one side to the same point on the op- 



