274 THE BRAIN OF THE TIGER SALAM.\NDER 



live, and the following summary is based on data from both species 

 (Amblystoma, figs. 6, ^25, 71, 75; '^27, p. 285; '396, p. 534 and figs. 1, 

 79; Necturus, '336, pp. 173, 261 and fig. 14; '346). The medial bundle 

 is incompletely separable into dorsal and ventral fascicles connected, 

 respectively, with the dorsal and ventral parts of the hypothalamus. 

 Both fascicles have extensive connections with the preoptic nucleus, 

 and both contain descending and ascending fibers. Posteriorly of the 

 anterior commissure ridge, thick collaterals of fibers of both groups 

 of fascicles ascend to the habenula in tr. olfacto-habenularis of the 

 stria medullaris (p. 257; '396, fig. 79). 



The dorsal fascicles contain descending fibers arising in the septum 

 (tr. septo-hypothalamicus) and in the primordium hippocampi (pre- 

 commissural fornix, p. 254), also some components of the stria ter- 

 minalis system (p. 256). These are accompanied by some ascending 

 fibers the connections of which are not clear. 



The ventral fascicles contain secondary olfactory fibers from the 

 bulb and some of higher order from the ventral and medial sectors of 

 the anterior olfactory nucleus. These fibers are distributed to the 

 preoptic nucleus and ventral part of the hypothalamus and are ac- 

 companied by many preoptico-hypothalamic fibers. Slender filaments 

 of the nervus terminalis are spread among these fascicles for their 

 entire length. The large hypophysial tract arises from neurons which 

 are widely scattered throughout the hypothalamic field reached by 

 both the ventral and the dorsal fascicles of the ventral bundle. In the 

 floor of the preoptic recess there is a median fascicle of unmyelinated 

 fibers, among which are a few with myelin sheaths. This is tr. 

 preopticus (p. 244). 



Very slender unmyelinated fibers arise from small cells at the ex- 

 treme posterior end of the ventral hypothalamus and ascend for 

 undetermined distances in the ventral fascicles. There are doubtless 

 other ascending fibers, but their courses have not been recorded. 



Olfacto-peduncular tract. — This is a well-defined round bundle, 

 lying between the lateral and the medial bundles and less myelinated 

 than the former (figs. 18, 21, 25-30, 53, 54, 59, 72, 95-99, 101; '27, 

 p. 286; '36, p. 336; '396, p. 534, figs. 1, 79, 80). Its fibers arise from 

 the head of the caudate nucleus and neighboring parts and dis- 

 tribute to the dorsal part of the hypothalamus, ventral part of the 

 peduncle, and interpeduncular nucleus. The hypothalamic connec- 

 tion allies this tract with the dorsal fascicles of the medial forebrain 

 bundle, the peduncular connection with the ventral fascicles of the 



