DISSECTION OF THE BRAIN, OE ENCBPHALON. 251 



ventricular aspect of the body is white, but the surface that rests on the 

 optic thalamus is grey. The hippocampus is to be viewed as a convolu- 

 tion of the cerebrum, being, in fact, an inward projection of the hippo- 

 canipal convolution already noticed (page 248). 



Directions. — The corpus callosum and septum lucidum should be cut 

 away in order to see the upper surface of the fornix, which shovild then 

 be divided transversely in its middle. The anterior part should be 

 raised forwards and upwards to expose its anterior pillars and the 

 foramen /ff Monro. The posterior part should be removed along with 

 the hijJpocampus in order to bring into view the velum interpositum. 

 The dissection will then assume the form of Plate 36 ; but the optic 

 thalami and the pineal gland, there exposed, will be covered by the 

 velum. 



The Velum Interpositum is a triangular fold of pia mater, continuous 

 by its base with the pia mater on the hinder end of the cerebrum. Its apex 

 lies at the foramen of Monro, and its lateral edges, fringed by the chor- 

 oid plexuses, project towards the lateral ventricle through what is termed 

 the great transverse fissure of the cerebrum. This is an arched cleft extend- 

 ing over the optic thalami, from the extremity of the descending horn on 

 ■" ae side to the same point on the other. Above it is bounded centrally 

 oy the fornix, and on each side by the hippocampus and its taenia. 

 The velum interpositum is, like the pia mater in general, a vascular 

 membrane ; and the choroid plexus of each side is a thickened 

 and highly vascular portion of it. Along its centre the veins of Galen 

 extend backwards, and unite to turn roimd the posterior extremity of 

 the corpus callosum, and enter the inferior longitudinal sinus. The 

 velum shovild now be raised from its apex backwards, when it will be 

 seen to cover the optic thalami and the pineal gland, and care must be 

 taken lest the latter be removed with it. 



The Optic Thalami. Each of these is a large grey-coloured body, 

 superposed to the cms cerebri behind the corpus striatum, and in front 

 of the corpora quadrigemina. Its upper surface is convex and covered 

 by the velum interpositum. When followed outwards this surface 

 changes its direction, looking backwards and downwards ; and it there 

 forms part of the boundary of the descending horn of the lateral 

 ventricle. Inwardly the right and left thalami are opposed to 

 one another along the middle line, and they include between them 

 the 3rd ventricle. In front each thalamus is separated from the 

 corpus striatum by a groove, in which will now be seen more distinctly 

 the taenia semicircularis. , Behind, another groove isolates the thalamus 

 from the nates. 



The. Pineal Gland is a small, reddish, conical body, named from its 

 resemblance to a pine cone. It stands by its base on the middle line 

 between the optic thalami and the nates. From its base two white 



