300 NERVOUS SYSTEM OF VERTEBRATES. 



seen in Figure 147 behind the canal leading to the recessus neu- 

 roporicus. The basal portion of the lateral wall is occupied by 

 the corpus striatum. The outer layers of the lateral wall and of 

 the thick roof constitute the lateral oljactory nucleus. The inner 

 surface of both the striatum and the lateral olfactory nucleus is 

 covered by a gray layer rich in small cells which bounds the ven- 

 tricle both ventro-laterally and dorsally. This is the epistriatum. 

 The course of the fiber tracts will show the functional signifi- 

 cance and homology of the various parts of the selachian brain. 

 The olfactory tract spreads through the whole anterior and lateral 

 wall and ends in the mesial and lateral olfactory nuclei. From 

 each of these nuclei a tract runs to the hypothalamus. The first 

 gathers from all parts of the mesial nucleus, curves downward and 

 backward in the thick front wall, runs back near the mid- ventral 

 line and joins the tractus strio-thalamicus. This tract should be 

 called the tractus olfacto-hypothalamicus medialis. The second 

 tract collects from the lateral olfactory nucleus, runs downward 

 and backward through the lateral wall, passes over the optic 

 chiasma, forms a decussation behind the chiasma and ends in the 

 hypothalamus. This tract has usually been called the pallial 

 tract because the nucleus from which it arises forms the roof of 

 the forebrain. This nucleus, however, is the place of ending of 

 olfactory tract fibers and is hence merely a part of the olfactory 

 lobe. The tract in question should therefore be called the tractus 

 oljacto-hypothalamicus later alls. From the lateral part of the 

 lateral olfactory nucleus another tract arises which runs along the 

 dorsal border of the narrow lateral wall of the posterior part of 

 the forebrain and enters the nucleus habenulae. Here the tract 

 decussates with its fellow to form the habenular commissure and 

 ends in the nucleus habenulae of the opposite side. This is the 

 tractus olfacto-habenularis and corresponds to the tract of the 

 same name in cyclostomes. That part of the lateral olfactory 

 nucleus from which it arises may be called the nucleus thaeniae. 

 The two tracts to the hypothalamus correspond to the one in 

 cyclostomes, the existence of a separate medial tract in selachians 

 being due to the extraordinary development of the olfactory 

 apparatus. 



