1 84 



NERVOUS SYSTEM OF VERTEBRATES. 



bulb of the same side or from that of the opposite side, crossing 

 in the anterior commissure. The fibers which arise from the 

 nuclei form two main tracts, the tractus oljacto-hypothalamicus 

 and the tractus oljacto-habenularis. The fibers of the former 

 tract arise from the cells of both portions, run backward through 

 the lateral and ventral parts of the corpus striatum and at the 

 junction of the forebrain and thalamus appear in a medial and a 

 lateral bundle. These enter the diencephalon and end in the 

 hypothalamus. The tractus olfacto-habenularis is smaller. It 

 arises chiefly from the nucleus thaeniae and sometimes from the 

 other olfactory nuclei including the nucleus praeopticus in the 

 walls of the preoptic recess, and runs upward and backward 

 through the epistriatum to the dorsal part of the diencephalon. 

 There each tract wholly or partially crosses to the opposite side, 



Tr. olfacto-haben 



Tr. strio-thalam 



Fiber of olfac 

 tory tract. 



Lateral olfactory 

 nucleus. 



Fig. 99. — A transverse section of the brain of the sturgeon at the level of the 

 anterior commissure. 



the decussation of the two tracts forming what is known as the 

 superior or habenular commissure. The tracts end in the nuclei 

 habenulae. These two tracts are tertiary olfactory tracts. 



Another secondary nucleus in the forebrain is to be described. 

 The central gray matter surrounding the forebrain ventricle, 

 bounded in front by the postolfactory nuclei and covered laterally 

 and ventrally by the lateral olfactory nucleus and the corpus 

 striatum, is known as the epistriatum. The epistriatum is com- 

 posed of pyramidal cells arranged in rows, the cell-bodies being 

 near the ventricle and the dendrites directed toward the surface. 



