350 



THE BRAIN OF THE TIGER SALAMANDER 



Figure 55. — Detail of the spiral course of fibers of the f . retroflexus below the decussation, 

 from a horizontal Golgi section of a late larva. X 50. In the interpeduncular neuropil nothing 

 but these three fibers is impregnated, so there is no possibility of confusion. 



Figures 56, 57, 5S.- — Three horizontal Golgi sections of an adult brain, in which the right 

 f . retroflexus is unstained and the left fasciculus is abundantly impregnated from the habenula 

 to its decussation. X 50. Some details of the decussation are added to figure 58 from the section 

 adjoining it ventrally. The impregnation fails spinalward of the decussation. The left fasciculus 

 shows an atypical division into two bundles as it enters the alba of the peduncle (p. 262). 



Figure 59.- — An obliquely horizontal section through the superficial origins of the III roots, 

 advanced larva. X 50. Here it is in about the plane of figure 30, but sharply inclined to the 

 horizontal plane, with the posterior end more ventral and the anterior end more dorsal than 

 that level. Impregnated fibers of the olfacto-peduncular tract pass the region of the fovea isth- 



mi and then turn medially. Most of them end in the interpeduncular neuropil with open 

 arborizations, though some extend farther spinalward (compare figs. 53, 54). From this neu- 

 ropil slender axons of the dorsal interpedunculo-bulbar tract descend near the mid-plane and 

 soon turn laterally to end in a dense axonic neuropil at the ventral border of the caudal end of 

 the tegmentum isthmi and rostral end of the tegmentum trigemini. Here they engage dendrites 

 of the smaller neurons of this region. Other preparations show that some of these fibers extend 

 spinalward as far as the IX nerve roots. 



Figures 60 to 6^. — These drawings present additional details from a series of transverse 

 Golgi sections (no. 2246), which has already been quite fully described and illustrated in the 

 sector of the brain stem between the interventricular foramen and the nucleus of the IV nerve 

 ('27, pp. 271, 278, figs. 22-40). Those figures were drawn from sections selected from nos. 

 55-97 of the series. Section 88 was subsequently drawn on a larger scale to show some details 

 of structure, including the arrangement of the tegmental fascicles, at the level of the III nerve 

 roots ('42, fig. 44). Figures 60-64 extend the series spinalward (X 50), with special reference to 

 the interpeduncular connections of the tegmentum, which are here well impregnated. These 

 tegmento-interpeduncular fibers comprise one component of the complex f. tegmentalis pro- 

 fundus (p. 286), and the incomplete references to them made in 1927 can now be clarified and 

 rectified. In this specimen there is no impregnation of the f . retroflexus or tr. olfacto-peduncu- 



