242 THE BRAIN OF THE TIGER SALAMANDER 



the hypothalamus are a few which separate from the optic tracts and 

 ramify in the vicinity of the chiasma (p. 221). 



By far tlie larger part of the white substance of the hypothalamus 

 is occupied by the great medial forebrain bundles (f.med.t. of the 

 figures); and, since these bundles are composed chiefly of fibers de- 

 scending from the olfactory field of the hemispheres, the implication 

 is that olfactory functions are dominant here. The olfacto-peduncular 

 tract arises chiefly from the head of the caudate nucleus and distrib- 

 utes some of its fibers to the dorsal part of the hypothalamus. Fibers 

 descend in the dorsal fascicles of the medial forebrain bundle from 

 the septum and primordium hippocampi and distribute chiefly to the 

 preoptic nucleus and dorsal hypothalamus. Included here are the pre- 

 commissural fornix and part of the stria terminalis system. The 

 ventral fascicles have descending fibers from the olfactory bulb and 

 from the ventral and medial sectors of the anterior olfactory nucleus, 

 which spread throughout the preoptic nucleus and the ventral part 

 of the hypothalamus. Strong collaterals of both dorsal and ventral 

 fascicles enter the stria medullaris thalami. Many fibers descend from 

 the preoptic nucleus in diffuse formation, to spread throughout the 

 hypothalamus posteriorly of the chiasma ridge. One component of 

 this preoptico-hypothalamic connection is the compact tr. preopti- 

 cus, as described below. 



The dorsal olfactory projection tract (ol.p.tr.) is a compact fascicle 

 of unmyelinated fibers, which descend from the dorsal striatal nu- 

 cleus and in larger number from the amygdala. This fascicle accom- 

 panies the lateral forebrain bundle, and above the chiasma it turns 

 ventrad to connect with its nucleus at the posterior border of the 

 chiasma ridge. This tract also contains ascending fibers. It was first 

 described in Ambly stoma and the frog ('21a) and subsequently in 

 Necturus ('336, p. 158) ; its course was fully illustrated in 1921 and is 

 shown in many other figures (figs. 19, 25, 26, 27, 95, 96, 97, 101, 102, 

 103; '27, figs. 11-20, 24-32; '36, fig. 5). 



Afferent fibers of the visceral-gustatory system enter the dorsal 

 part of the hypothalamus (fig. 8) . The ventral part receives fibers 

 from the pretectal nucleus by tr. pretecto-hypothalamicus (figs. 15, 

 16), and much larger numbers by the accompanying tr. thalamo- 

 hypothalamicus, and also by tr. thalamo-hypothalamicus cruciatus 

 (fig. 16). The entire tectum is connected with the hypothalamus by 

 two large tracts — (1) the anterior division of tr. tecto-thalamicus et 



