DISSECTION OF THE BRAIN, OR ENCBPHALON. 257 



The Corpus Striatum (Plates 35 and 36). This is the large grey 

 body already noticed in the body and anterior cornu of the lateral 

 ventricle. In shape it is pyriform, having its broad end directed 

 forwards and inwards, and its tapering end backwards and outwards to 

 the roof of the descending cornu. The body comprises two masses of 

 grey matter, separated from each other by intermediate white fibres 

 which curve upwards and outwards from the cerebral orus. The upper 

 mass of grey matter — termed the nucleus oaudatits — is that which pro- 

 jects into the lateral ventricle. The lower mass — ^the nucleus lenticu- 

 laris — lies above the quadrilateral space already seen at the base of the 

 hemisphere between the diverging olfactory tracts. 



The TAENIA Semioircularis (Plate 36) is a narrow white band that 

 extends between the corpus striatum and the optic thalamus. (Its 

 relation to the thalamus will be better seen in the next stage of the 

 dissection.) 



The Hippocampus is the curved eminence already noticed in the body 

 and descending horn of the lateral ventricle. It rests on the optic 

 thalamus, from which it is separated by the velum interpositum. The 

 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- 

 campal convolution already noticed (page 254). 



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

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

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

 raised forwards and upwards to expose its anterior pillars and the 

 foramen of Monro. The posterior part should be removed along with 

 the hippocampus 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 

 choroid plexuses, project towards the lateral ventricle through what is 

 termed the great transverse fissure of the cerehnim. This is an arched 

 cleft extending over the optic thalami, from the extremity of the 

 descending horn on one side to the same point on the other. Above 

 it is bounded centrally by the fornix, and on each side by the hippo- 

 campus 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 

 round the posterior extremity of the corpus callosum and enter 

 the inferior longitudinal sinus. The velum should now be raised 



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