THE OLFACTORY APPARATUS. 



185 



A large number of fibers of the olfactory tract which decussate 

 in the anterior commissure end in the epistriatum. As has been 

 mentioned (p. 181), the epistriatum receives also fibers from the 

 hypothalamus, which probably in large part or wholly carry 

 gustatory impulses. The epistriatum is thus related to both 

 the sense organs which are concerned with the food relations of 

 the organism. A large part of the epistriatum cells have short 

 neurites which end in the underlying corpus striatum (Fig. 149) 

 from which the tractus strio-thalamicus goes backward. This 

 constitutes a third path by which olfactory impulses may reach 

 the diencephalon. As the result of the distribution of these three 

 tracts the greater part of the gray matter surrounding the third 

 ventricle is brought into the service of the olfactory organ. 

 In selachians, where the olfactory apparatus reaches an unus- 



Tr. pinealis I 

 Tr. pinealis 11 



tectum 



Post.comm 

 Tr. hab-pedunc. 



Commissura superior 



Comm. ant 



Fig. 100. — An outline of the median sagittal section of the forebrain of Lampetra. 



ually great development, the nuclei and commissures require some 

 explanation. In all lower vertebrates the olfactory portion of 

 the anterior commissure is more or less distinct from the portion 

 which is formed of fibers coming forward from the hypothalamus. 

 In Petromyzon the olfactory commissure is quite separate and 

 is situated farther forward and upward on the front end of the 

 forebrain (Fig. 100). In selachians the dorsal and lateral olfactory 

 nuclei extend up from the front and side walls to form the thick 

 nervous roof of the forebrain which has been mentioned. No 



