THE OPTIC THALAMl'S. 760 



ate bundles, between which the grey matter is continued from 

 the caudate to the lenticular nucleus. 



On the outer side of the external capsule, separating it from the Claustrum. 

 medullary substance of the convolutions of the insula, is a third 

 portion of grey matter, which appears, in sections, as a slightly 

 wavy grey line : this is named the claustrum (fig. 278, cl ; and 

 fig. 279), and it represents an enlarged and well-defined fifth layer 

 of the cerebral cortex. - 



The T.^KNIA SEMICIRCULARIS (fig. 275, /) is a narrow whitish Taenia senri- 



band of longitudinal fibres, which lies along the groove between the C11 

 caudate nucleus and the optic thalamus. In front, the band 

 bf comes broader and joins the pillar of the fornix ; behind, it is 

 continued with the tail of the caudate nucleus into the roof of the 

 descending cornu of the lateral ventricle, at the lower end of which 

 it joins the amygdaloid nucleus. 



Dissection. The anterior commissure is next to be exposed in Anterior 

 its course through the cerebral hemisphere. For this purpose the *j 

 remaining fore part of the caudate nucleus, the white fibres, and to show it ; 

 the lenticular nucleus, on the right side, must be successively 

 ^craped away with the handle of the scalpel, and the rounded band 

 traced outwards from the spot where it is seen at the front of the 

 third ventricle to the medullary centre of the temporo-sphenoidal 

 lobe. 



The ANTERIOR COMMISSURE is a round bundle of white fibres its form, 

 about as large as a crow-quill, which is free only for about an eighth 

 of an inch in the middle of its extent, where it lies in front of the position, 

 pillars of the fornix (fig. 279). Laterally, it passes outwards 

 beneath the corpus striatum, lying between the lenticular nucleus course, and 

 and the grey matter of the anterior perforated space, and curving 

 backwards, spreads out in the white substance of the temporal lobe ending, 

 above the descending cornu of the lateral ventricle. 



The OPTIC THALAMUS (fig. 277, 6) is an oval-shaped body which Optic 

 takes part in bounding the lateral and third ventricles. Its upper th 

 surface is marked by a shallow oblique groove, which corresponds upper 

 to the edge of the fornix. The part of the surface inside the surface ; 

 groove is in contact with the velum interposition ; but the 

 narrower outer part is free in the floor of the lateral ventricle, and 

 is covered by the ependyma of that cavity : at its anterior end it 

 forms a slight prominence known as the tubercle of the optic 

 thalamus. Externally this surface is bounded by the tsenia semi- 

 circularis, which separates it from the caudate nucleus. The inner inner 

 surface is for the most part free, forming the lateral wall of the surface : 

 third ventricle, but near the middle it is united to the one of the 

 opposite side by the middle commissure (d). Along the line of 

 junction of the upper and internal surfaces is a narrow white 

 streak the pineal stria (/), which springs behind from the stalk 

 of the pineal body, and ends in front by joining the anterior pillar 

 of the fornix. 



The under surface is concealed, except at its hindmost part, by lower and 

 the cms cerebri, the tegmentum of which joins the thalamus ; and surfaces ; 



D.A. 3 D 



