310 NERVOUS SYSTEM OF VERTEBRATES. 



tractus olfacto-habenularis receives fibers from the dorso-caudal 

 pole of the hemisphere. These fibers constitute a tractus cortico- 

 habenularis. A commissure is found in Hzards and some other 

 reptiles which connects the dorso-caudal poles of the hemispheres 

 directly, above the ventricle. This commissure runs in the velum 

 transversum and does not correspond in position to any com- 

 missure found in other vertebrates. It is therefore called the 

 comniissura aberrans (Fig. 158). The position and extent of the 

 several parts of the forebrain are shown in Figures 155, 156 and 

 157. The last figure does not show the whole extent of the hip. 



Fig. 157. — A diagram of the mesial surface of the hemisphere of a reptile to 

 show the extent of the hippocampus and related structures. The lamina terminalis 

 is shaded with horizontal lines and in it are shown the anterior and hippocampal 

 commissures. The hippocampal area is shaded with oblique lines, b, bulbus 

 olfactorius; p, paraterminal or precommissural body (septum). 



pocampus but shows the position of its mesial part relative to the 

 lamina terminahs, the foramen of Monro, the commissures and 

 the mesial olfactory nucleus (septum). 



The mammalian forebrain. — In monotremes and marsupials 

 the relations of the olfactory centers are essentially as in amphibia 

 and reptiles, although there is a higher development in internal 

 structure. The olfactory bulbs and lobes are relatively small 

 (Figs. 159, 160) and the rest, of the hemisphere very large in 

 proportion. Indeed, in monotremes and marsupials as in mam- 

 mals, the lateral and dorsal walls of the hemispheres do not belong 

 to the olfactor}^ apparatus as in reptiles and lower forms, but 



