BASE OF THE BRAIN. 377 



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 extremity of the corpus cal- 

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



On each side of the longitudinal fissure, upon the under surface of 

 each anterior lobe, 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 convolutions will be observed ; these constitute the island of Reil. 



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 

 arteries, which enter the brain in this situation to supply the gray 

 substance 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 

 extends beneath the hemisphere of one side to the same point on the 

 opposite side. 



The Optic commissure is situated on the middle line ; it is the point 

 of communication between the two optic nerves. 



The Tuber cinereum is an eminence of gray substance immediately 

 behind the optic commissure, and connected with its posterior bor- 

 der. It forms part of the floor of the third ventricle. 



The Infundibulum is a tubular process of gray substance, opening 

 from the centre of the tuber cinereum, and attached below to the 

 pituitary gland, which is lodged in the sella turcica. This gland is 

 retained within the sella turcica by the dura mater and arachnoid, 

 and is with great difficulty removed with the brain. It is, therefore, 

 better left in its place, where it is intended to study afterwards the 

 base of the skull; for any attempt at removal would injure the 



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