BASE OF THE BRAIN. 383 



dissection have hitherto been conducted with care, he will find the base 

 perfectly uninjured. The arachnoid membrane, some parts of the pia 

 mater, and the circle of Willis, must be carefully cleared away, in order 

 vo expose all the parts to be examined. These he will find arranged in 

 i^ following order from before backwards : 



Longitudinal fissure, Infundibulum, 



Olfactory nerves, Corpora albicantia, 



Fissure of Sylvius, Locus perforatus, 



Substantia perforata, Crura cerebri, 



Commencement of the trans- Pons Varolii, 



verse fissure, Crura cerebelli, 



Optic commissure, Medulla oblongata. 

 Tuber cinereum, 



The Longitudinal fissure is the space separating the two hemispheres : 

 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 callosum will be seen descending to 

 the base of the brain. Arrived at the base of the brain, the corpus callo- 

 sum terminates by a concave border, which is prolonged to the commis- 

 sure 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 cal- 

 losum is continued into the substantia perforata and crura cerebri, and 

 upon the latter forms a narrow medullary band lying externally to, and 

 slightly overlapping the optic tract, the medulla innominata. 



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

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



The Fissure of Sylvius bounds the anterior lobe posteriorly, and sepa- 

 rates 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 island of Reil. The 

 island of Reil, together with the substantia perforata, form the base of the 

 corpus striatum. 



The Substantia perforata is a triangular plane of white substance, situ- 

 ated at the inner extremity of the fissure of Sylvius. It is named perfo- 

 rata from being pierced by a number of openings for small arteries, which 

 enter the brain in this situation to supply the grey 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. A 

 probe passed into this fissure between the crus cerebri and middle lobe 

 would enter the middle cornu of the lateral ventricle. 



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 grey substance situated immedi- 

 ately behind the optic commissure, and in front of the corpora mam-mii- 

 laria. From its centre there projects a small conical body of grey sub- 

 stance, apparently a prolongation of the tuber cinereum, the infundibulum 

 The infundibulum is hollow in its interior, enclosing a short caecal 



