CORPUS CALLOSUM. 411 



brain, terminating by each extremity at the fissure of Sylvius. The 

 sulcus between this convolution and the corpus callosum has been 

 termed, very improperly, the " ventricle of the corpus callosum," and 

 some longitudinal fibres (striae longitudinales laterales), which are 

 brought into view when the convolution is raised, were called by 

 Reil the "covered band." If, now, the upper part of each hemisphere 

 be removed to a level with the corpus callosum, a large expanse of 

 medullary substance, surrounded by a zigzag line of grey substance 

 corresponding with the convolutions and sulci of the two hemispheres, 

 will be seen ; this is the centrum ovate majus of Vieussens. 



The Corpus callosum (callosus, hard) is a thick layer of medullary 

 fibres passing transversely between the two hemispheres, and con- 

 stituting their great commissure. It is situated in the middle line 

 of the centrum ovale majus, but nearer to the anterior than to the 

 posterior part of the brain, and terminates anteriorly in a rounded 

 border (genu), 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 (splenium), which is continuous with the 

 fornix. The breadth of the corpus callosum is about four inches. 



Beneath the posterior rounded border of the corpus callosum is the 

 transverse fissure of the cerebrum, which extends between the hemi- 

 spheres and crura cerebri from very near the fissure of Sylvius on one 

 side, to the same point 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 (chordae 

 longitudinales Lancisii) ; and, on either side of the raphe, may be 

 seen the linecs transverse, which mark the direction of the fibres 

 of which the corpus callosum is composed. These fibres may be 

 traced into the hemispheres on either side, and they will be seen to 

 be crossed at about an inch from the raphe by the longitudinal fibres 

 of the covered band of Reil. Anteriorly and posteriorly the fibres of 

 the corpus callosum curve into their corresponding lobe. 



If, now, a superficial incision be made through the corpus callosum 

 on either side of the rapht, 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, their 

 upper boundary should be removed with the scissors. In making this 

 dissection the thin and diaphanous membrane of the ventricles may 

 frequently be seen. 



LATERAL VENTRICLES. Each lateral ventricle is divided into a 

 central cavity, and three smaller cavities called cornua. The anterior 

 cornu curves forwards and outwards in the anterior lobe ; the middle 

 cornu descends into the middle lobe ; and the posterior cornu passes 

 backwards in the posterior lobe, converging towards its fellow of the 

 opposite side. The central cavity is triangular in its form, being 



