LATERAL VENTRICLES. 



373 



The Corpus callosum (callosus, hard) 

 is a thick layer of medullary fibres 

 passing transversely between the two 

 hemispheres, and constituting their 

 great commissure. It is situated in the 

 middle line of the centrum ovale ma- 

 jus, but nearer the anterior than the 

 posterior part of the brain, and termi- 

 nates anteriorly in a rounded border 

 (genu), which may be traced down- 

 wards 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 length of the 

 corpus callosum is about four inches. 



Beneath the posterior rounded bor- 

 der of the corpus callosum is the trans- 

 verse fissure of the cerebrum, which 

 extends bet\veen the hemispheres and 



crura cerebri from 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 de- 

 pression between two slightly elevated longitudinal bands (chordas longi- 

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

 linea transversa, which mark the direction of the fibres of which the cor- 

 pus callosum is composed. These fibres may be traced into the hemi- 

 spheres 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 lobes. 



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

 either side of the raphe, two irregular cavities will be opened, which ex- 

 tend 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 de- 

 scends 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 form, being bounded above (roof) by 

 the corpus callosum ; internally by the septum lucidum, which separates 



* A section of the brain showing the centrum ovale majus and corpus callosum 

 1, 1. The anterior lobes of the brain. 2, 2. The posterior lobes. 3, 3. The longitudinal 

 fissure for the reception of the falx cerebri. 4, 4. The roof of the lateral ventricles. 

 5, 5. The genu of the corpus callosum. 6. Its body, upon which the linese transversal 

 are seen 7, 7. The splenium corporis callosi. 8. The raphe. 9, 9. The striae longi 

 tudinales laterales, or covered bands of Reil. 



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