980 OPTIC THALAMUS. [BOOK m. 



the so-called transverse fissure of the brain, shewn shaded in the figure, by 

 which the pia mater passing on beneath the posterior part of the cerebrum and 

 above the cerebellum gains access to the third ventricle, the position of the 

 velum being shewn by the thin black line stretching from the splenium to the 

 fornix. The front (and more ventral) part of the third ventricle is seen to end 

 in the infundibulum attached to which is the pituitary body H, seen in section 

 at L. In front of the infundibulum is seen the optic nerve cut across at the 

 optic decussation OP, stretching from which to the anterior commissure is the 

 lamina terminalis. 



Stretching between the corpus callosum c.c. (seen in longitudinal section with a 

 striated appearance and ending in front at the rostrum R. and behind at the 

 splenium Sp. ) dorsally and the fornix ventrally is seen (unshaded) the septum 

 lucidum S.L, but the greater part of this has been cut away in order to disclose 

 the right lateral ventricle in the wall of which is seen the bulging nucleus 

 caudatus N.C. 



Above the corpus callosum is seen the mesial surface of the right hemisphere 

 forming the right lateral wall of the longitudinal fissure. On this mesial 

 surface appears immediately above the corpus callosum the arched gyrus 

 fornicatus G.F, defined above by the calloso-marginal fissure /.cm. The whole 

 of the surface seen in the frontal region in front of the calloso-marginal fissure, 

 though divided by fissures, is called the marginal convolution. In the middle 

 parietal region a block of the cerebral substance has been removed in order to 

 shew the position of the central fissure or fissure of Rolando, f.c., and 

 immediately below this is seen a part of PA.C the paracentral lobule. In 

 the occipital region PR. C. is the precuneus or quadrate lobule, and C, the 

 cuneus, while at G.L is seen a part of the lingual lobule. T.i is a part of the 

 inferior temporo -occipital convolution, the greater part of which is hidden to 

 view by the pons and crus. 



above, and the diverging crus below. Its more or less straight 

 lateral border abuts on the internal capsule (Figs. 115, 116, 121). 

 Its dorsal surface, as we have already seen, also forms part of the 

 wall of the third ventricle and is free ; but there lies close above 

 it the prolongation of the pia mater, forming the velum inter- 

 positum with its choroid plexus ( 602), which creeps in over it 

 beneath the projecting hind end of the corpus callosum and 

 the fornix (Fig. 120). Its ventral surface is fused with the crus; 

 indeed the tegmental or dorsal portion of the crus may be said 

 to end in it and in certain structures lying ventral to the 

 thalamus, in what is called the " subthalamic region" (Fig. 116), 

 while the fibres of the pes pass first ventral and then lateral to 

 it to form the internal capsule. 



The grey matter of the whole body is more or less distinctly 

 divided by sheets of white matter, as seen both in horizontal and 

 in vertical sections (Figs. 115, 116, 121), into three parts which 

 have received the name of nuclei, namely the median or inner 

 nucleus (Fig. 116, in) which with the thin layer of central grey 

 matter forms the side wall of the third ventricle, the larger lateral 

 nucleus (In) which abuts upon the internal capsule, and the small 

 anterior nucleus (an) which lies on the dorsal surface of the front, 

 part of the body and which thus at its front end appears to project 

 into the lateral ventricle. 



These three nuclei form however not the whole of the optic 

 thalamus, but only the larger front portion ; behind them lies the 



