THE EVOLUTION OF THE CEREBRAL HEMISPHERES. 317 



constitute the somatic pallium. The olfactory cortex is now 

 confined to a part of the mesial wall of the hemisphere. Its 

 extent is best seen in a view of the mesial surface of the brain 

 (Fig. 1 60). The dorsal part of the mesial wall belongs with the 

 dorsal wall to the general cortex or neopallium whose functions 

 are chiefly somatic. The region corresponding to the lateral 

 olfactory nucleus of lower vertebrates is crowded down upon the 

 ventral surface of the hemisphere and forms the pyrijorm lobe 

 (Fig. 159). This is separated from the general cortex by the 

 fissura rhinalis. The mesial olfactory nucleus is in the same 



Fig. 158. A mesial sagittal section of the brain of an embryo of Sphenodon 

 punctatum. From G. Elliot Smith (Aberrant Commissure, etc.). a. S., aqueduct 

 of Sylvius; b.o., bulbus olfactorius; c.a., commissura aberrans; c.d., hippocampal 

 commissure; c.h., habenular commissure; c.p., posterior commissure; c.v., anterior 

 commissure; ce., cerebral hemisphere; hyp., hypophysis; l.t., lamina terminalis; 

 opt., tractus opticus; par., paraphysis; p.o., olfactory peduncle; p.s., parietal 

 stalk; plx.II.., lamina chorioidea; tub'., tuber cinereum; v.III., third ventricle. 



position as in lower vertebrates, forming the tuberculum olfactorium 

 and the lower portion of the mesial wall called the precommissural 

 body (G. Elliot Smith). In the lamina terminalis is a very large 

 anterior commissure and above it in the dorsal border of the 

 lamina terminalis is a smaller hippocampal commissure. This 

 commissure crosses in front of the median ventricle and enters 

 the hippocampus above the foramen of Monro, and therefore 

 corresponds in position to the hippocampal commissure of reptiles. 



