38 Journal of Comparative Neurology. 



are very few in number. On the other hand, a bundle of con- 

 siderable size comes out from the postoptic decussation and 

 runs dorsad, crossing the internal surface of the optic tracts, 

 and enters the epistriatum where its end-branching forms a very 

 rich network of fine fiber twigs (Figs. 17 a, 18, 30). This rep- 

 resents the chief part of the ascending fibers of the tractus 

 strio-thalamicus of other fishes. Its runing by way of the post- 

 optic decussation is peculiar to Petromyzon. Whether this tract 

 crosses in the postoptic decussation or anterior commissure, it 

 has the same connections and functions and should be called 

 the iractiis lobo-epistriaticiis. The descending fibers from the 

 olfactory nuclei to the hypothalamus, usually assigned to the 

 tractus strio-thalamicus, come from the lateral expansions of the 

 fore brain and from the nucleus thaeniae, and end in all parts 

 of the inferior lobes and corpus mammillare (Figs. 18, 19, 20). 

 They should be given the name tractits olfacto-lobaris. 



The saccus is very much smaller than in Acipenser and its 

 study has been correspondingly difficult. In Golgi sections no 

 nerve or sense cells have been impregnated, but a few cells 

 which show a close resemblance to the ependyma cells are 

 stained. Nerve fibers running into the saccus from the inferior 

 lobes have been stained in a number of preparations (Figs. 

 17 a, 18). Their internal connections can not be found in 

 Golgi sections, but in haematoxylin preparations the fibers are 

 easily followed forward along the lateral angle of the postoptic 

 recess to the level of the postoptic decussation when they bend ab- 

 ruptly dorsad and pierce the decussation to reach the central 

 grey surrounding the ventral part of the cavity of the thalamus. 

 This is the same region from which one of the saccus bundles in 

 Acipenser arises, but it was not traced so clearly thereas in Petro- 

 myzon. The bundle in Acipenser which has this course ends 

 freely in the saccus, and it is to be concluded that the bundle in 

 both forms arises from the central grey of the thalamus just above 

 the chiasma, runs as a paired bundle along the ventral wall of 

 the inferior lobes and ends freely among the cells of the saccus 

 epithelium, its probable function being to control the secretory 

 action of this membrane. The second saccus bundle of Aci- 



