294 



NERVOUS SYSTEM OF VERTEBRATES. 



The lateral lobes are rounded and are divided into anterior and 

 posterior portions by a slight groove on the lateral surface, nearly 

 vertical in position. If the brain be cut into halves in the median 

 sagittal plane and one half looked at from the mesial surface 

 (Fig- 145) the relation of all the parts can be better seen. In 

 such a hemisection there is seen in the dorsal wall of the dien- 



Olf. decuss 



Comm. habenularis 

 Tr. olfacto-haben. 

 Epistriatum 



Tr. haben.-pedunc. Comm. post. 



Tr. olfact. 



Comm. anter." 

 Nuc. praeoptic 



Tr. oH.-hypothal / 

 Optic chiasma 



Tr. lobo- 



epistriaticus 



Fig. 145. — A diagram of the fiber tracts in the forebrain of a cyclostome, Lampetra 

 Wilderi. The mesial surface of the right half of the forebrain and interbrain is 

 drawn. The deeper shading indicates the wider parts of the ventricle and the dark 

 oval opening is the foramen of Monro. The fiber tracts and parts of them which 

 lie farthest laterad are drawn in the darkest shade. 



cephalon the thick nucleus habenulae and opposite it ventrally 

 the optic chiasma and the decussations of other tracts. Behind 

 this is the depressed hypothalamus. The limit between the dien- 

 cephalon and telencephalon is roughly indicated by a hne drawn 

 between the optic chiasma and the nucleus habenulae. In front 



