284 NERVOUS SYSTEM OF VERTEBRATES. 



The impulses are carried by the neurites of the sense cells to the 

 thalamus. The fibers form symmetrical tracts which in the 

 sturgeon run up through the walls of the corpora mammillaria to 

 end in a nucleus adjacent to the nucleus of the tractus strio- 

 thalamicus in the extreme ventral part of the thalamus (Figs. 133, 

 134). In bony fishes the nucleus is situated lower down and a 

 part of the tract decussates in the caudal wall of the corpora 

 mammillaria (Goldstein). The tract in teleosts also arises from 

 the ciliated cells of the saccus (Johnston) and a similar tract is 

 present in selachians. The tract does not arise in the dorsal part 

 of the thalamus and end in the saccus as Edinger described it, 

 but arises in the saccus. The secondary connections of the end- 

 nucleus of this tract are not clear, but in teleosts a secondary tract 

 has been traced caudally over the ansulate commissure. 



Corpus mamillare 

 Tr. sacco-thalam. 



Saccus 



Fig. 144. A general scheme of the saccus tracts as projected upon the median 

 plane. 



In addition to the tractus sacco-thalamicus a tract goes out 

 from the hypothalamus to end in the saccus. This tract comes 

 in the sturgeon from the region just behind the optic chiasma (Fig. 

 143) and in the salamander the origin of its fibers from cells in 

 this position has been demonstrated (Bochenek). The fibers 

 of this tract go to all parts of the saccus vasculosus and ramify 

 richly among the cells of the epithelium. In the salamander 



