118 



NERVOUS SYSTEM 



thus giving rise to the uncus. Meanwhile the formation begins to be separated into 

 its several parts. The anterior extremity becomes cut off from the frontal lobe, 

 as a hollow stalk which communicates with the tip of the frontal horn of the 

 cavity. This separated stalk forms the olfactory bulb and tract as well as the 

 trigone. The remainder of the formation is not so isolated, and is represented by 

 the anterior perforated spot and the lobus pyriformis (fig. 160). This is a well- 

 marked swelling during the earlier months, extending from the olfactory tract to the 

 inner aspect of the temporal lobe, but it becomes greatly reduced, and is ultimately 



FIG, 162. CORONAL SECTION OF THE BRAIN OF A HUMAN EMBRYO OF 30 MM. (BEGINNING OF 

 THIRD MONTH). Photograph. (T. H. Bryce.) 



The section cuts the thalami, cerebral hemispheres, and corpora striata. H, H, posterior lobes of 

 hemisphere; /, /, frontal lobes. Ill is placed in the habenular region of the third ventricle; in front 

 of this the cavity is slit-like, and bounded by the bodies of the thalami. Each thalamus terminates in 

 a blunt-pointed end, the future anterior tubercle ; the anterior portion of the outer surface is oblique, 

 and separated from the overlapping corpus striatum by a fissure, in which the tsenia semicircularis is 

 afterwards developed. The infrachoroideal ependymal lamella (see especially on right side of figure) 

 of the mesial hemisphere-wall is continued over this surface and fused with it. The corpus striatum 

 being arched, is cut in two places, head (c 1 , c 2 ) and tail (t) ; between these, the commencing internal 

 capsule is seen. The head-end of the corpus striatum shows two crura, a mesial (c') and a lateral (c 2 ). 

 The longitudinal fissure between the frontal lobes is distorted by a large haemorrhage. On the left of 

 the figure the mesial wall shows a fold, which is the upper end of the fissura prima. 



represented only by the uncus and the so-called lateral root of the olfactory tract. 

 On the mesial aspect of the hemisphere a special rhinencephalic area trapezoid 

 plate or field (His), area paraterminalis (Elliot-Smith) is marked off by a distinct 

 fissure in front (fissura prima), and limited by the lamina terminalis behind 

 (fig. 164). Its fate will be considered later. 



On the mesial wall of the pallium the formation of the choroidal fissure has 

 already taken place. The vesicle-wall, where it passes over on to the thalamus- 

 wall, remains ependymal, and is folded into the cavity as a double layer enclosing 



