CORPUS STRIATfM 



119 



vascular pial tissue. Thus a plica choroidea is produced (fig. 150), which becomes 

 so extensive as nearly to fill the interior of the vesicle. The choroidal fissure 

 begins at the angle where the lamina terminalis and pallio-thalamic border of the 

 foramen of Monro meet. It is at first short and nearly straight, but gradually 

 extends round the hemisphere-stalk to the tip of the temporal horn. It is to be 

 observed that below the fissure there is an ependymal lamella (infrachoroideal 

 raembrane) which becomes fused with the thalamus (see below). 



It will be convenient at this stage to take the development of the corpus 

 striatum; As we have already seen, it lies in the floor of the hemisphere- 

 vesicle (figs. 156, 157) ; it is directly continuous, below and behind the primitive 



FIG. 1G3. SECTION THROUGH THE SAME BRAIN AS SHOWN IN FIG. 1G2, SEVERAL SECTIONS 



NEARER THE BASE. (T. H. BrVC6.) 



The same general description as given under fig. 162 will apply here, but it will be noted that the 

 fissure between the thalamus and corpus striatum is now interrupted by the large primary ihalamus- 

 bundle. This marks the stalk of the hemisphere, round which the corpus striatum arches ; as the 

 stalk enlarges, the fissure (also, of course, arched) is gradually obliterated, and is only represented in 

 the adult brain by the furrow between caudate nucleus and thalamus, in which the taenia semi- 

 circularis lies. The hollow on the surface of the hemisphere opposite the stalk is the fossa of Sylvius. 



foramen of Monro, with the thalamus, and is connected in front with the 

 rhinencephalon by three roots. The middle of these fuses with the internal fold 

 corresponding to the fissura prima (fig. 164) in the floor of the vesicle and cuts off 

 a pocket of the frontal horn which is continuous with the cavity of the olfactory 

 stalk. When the lumen of the stalk is obliterated this pocket disappears. The 

 cleft behind this seems in part obliterated in its basal portion, but persists 

 above as the space between the caudate nucleus and septum pellucidum in 

 the adult brain. The body of the corpus striatum grows backwards pari passu 

 with the development of the temporal horn of the vesicle, and its tail is 

 thus produced (fig. 150). As the hemisphere-rudiment becomes more and more 



