THE GRAY MATTER. 623 



on the internal capsule (Figs. 138, 139, 144). 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 interpositum with its choroid plexus ( 515), which creeps in over it 

 beneath the projecting hind end of the corpus callosum and the fornix 

 (Fig. 143). Its ventral surface is fused with the crus ; indeed the tegmental 

 or dorsal portion of the cms may be said to end in it and in certain struc- 

 tures lying ventral to the thalamus, in what is called the " subthalamic 

 region" (Fig. 139), while the fibres of the pes pass first ventral and then 

 lateral to it to form the internal capsule. 



The gray 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. 136, 139, 144), into three parts which have received the name of 

 nuclei, namely, the median or inner nucleus (Fig. 139 m), which with the 

 thin layer of central gray matter forms the side wall of the third ventricle ; 

 the larger lateral nucleus (ln\ which abuts upon the internal capsule ; and 

 the smaller 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 important portion 

 called the pulvinar, into which the hind part of the median nucleus merges; 

 this is partly imbedded in the crus ventrally, and in the hemisphere laterally, 

 and is partly free, coming to the surface beneath the hind end of the corpus 

 callosum. In a median longitudinal section of the brain (Fig. 143) it is the 

 pulvinar which forms the cushion-like (hence the name) end of the thalamus 

 beneath the overhanging splenium of the corpus callosum, by the side of the 

 pineal gland ; and in the horizontal view (Fig. 138, Pvr), in which the 

 hemispheres are supposed to have been removed, the same pulvinar is seen 

 projecting over the crus by the side of the anterior corpus quadrigeminum. 

 The buried portion of the pulvinar is exposed in a transverse section taken 

 through the anterior corpus quadrigeminum (Fig. 137) ; the extreme end of 

 this part of the pulvinar (Pvr) is here seen lying dorsal and lateral to the 

 pes of the crus, immediately above two masses of gray matter, the corpora 

 geniculata (Cgl. Cgm.), of which we shall speak later on. One of these, 

 the lateral corpus geniculatum ((7. g. /.), is especially connected with the 

 optic tract (op), and, as we shall see hereafter, the pulvinar itself is also 

 connected with the optic tract, and is an important part of the central appa- 

 ratus of vision. 



542. The substantia nigra, the red nucleus, and other gray matter of the 

 tegmentum. Nerve-cells and groups of nerve-cells, or areas of gray matter, 

 too small to deserve special names, are scattered throughout the tegmentum 



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 cc (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 SL, 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 NC. 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 GF, defined 

 above by the calloso-marginal fissure f. 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 

 show 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 from view by the 

 pons and crus. 



