CENTRUM OVALE MAJUS CORPUS CALLOSUM. 367 



fossa, in the base of the skull, and is separated from the posterior by 

 a slight impression produced by the ridge of the petrous bone. The 

 posterior lobe is supported by the tentorium. 



If the upper part of one hemisphere % be removed with a scalpel, a 

 centre of white surface will be observed, surrounded by a narrow 

 border of gray, which follows the depressions of the convolutions, 

 and presents a zigzag outline. This appearance is called centrum, 

 ovate minus. The divided surface will be seen to be studded with 

 numerous small red points (puncta vasculosa) which are produced 

 by the escape of blood from the divided ends of minute arteries and 

 veins. 



Now separate carefully the two hemispheres of the cerebrum, and 

 a broad band of white substance will be seen to connect them. Re- 

 move the upper part of each hemisphere, with a knife, to a level with 

 this white layer. The appearance resulting from this section is the 

 centrum ovale majus. 



The Centrum ovale majus is the large centre of white substance 

 presented to view on the removal of the upper part of both hemi- 

 spheres ; it is surrounded by the thin stratum of gray substance, 

 which follows in a zigzag line all the convolutions and the fissures 

 between them. In the middle of the centrum ovale majus is the 

 broad band which connects the two hemispheres to each other, the 

 corpus callosum. 



The Corpus callosum (callosus, hard) is a dense layer of transverse 

 fibres connecting 'the two hemispheres and constituting their great 

 commissure. It is situated nearer to the anterior than to the poste- 

 rior part of the brain, : and terminates anteriorly in a rounded border 

 which may be traced downwards to the base of the brain, in front 

 of the commissure of the optic nerves. Posteriorly it forms a thick 

 rounded fold which is continuous with the fornix. 



Beneath the posterior rounded border of the corpus callosum is 

 the transverse fissure of the cerebrum, which extends between the 

 hemispheres and crura cerebri from the fissure of Sylvius on one 

 side, to that on the opposite side of the brain. It is through this 

 fissure that the pia mater communicates with the velum interpositum. 

 And it was here that Bichat conceived the arachnoid to enter the 

 Ventricles; hence it is also named the fissure of Bichat. 



Along the middle line of the corpus callosum is the raphe, a linear 

 depression between two slightly elevated longitudinal bands; and, on 

 either side of the raphe, may be seen the linear transversa, which 

 mark the direction of the fibres of which the corpus callosum is 

 composed. 



If an incision be made through the corpus callosum on either side 

 of the raphe, two irregular cavities will be opened, which extend 

 from one extremity of the hemispheres to the other: these are the 

 lateral ventricles. To expose them completely the upper boundary 

 should be removed with the scissors. 



Each Lateral ventricle is divided into a central cavity, and three 

 smaller cavities called cornua. The anterior cornu curves forwards 



