280 THE BRAIN OF THE TIGER SALAMANDER 



thalamo-peduncularis et tegmentalis (figs. 21, 23), which contains 

 both descending and ascending fibers. In the first description these 

 were designated simply as "fibers from the postoptic commissure" 

 ('36, p. 304, figs. 3, 8, 19, po.{Jt-)); but now their connections are 

 better known ('42, p. 226 and fig. 3), thanks to the fact that they 

 mature very early in ontogeny and so can be seen in young stages, 

 despite their dispersed arrangement. A few elective Golgi impregna- 

 tions confirm these findings in the adult. 



Fibers related with the entire ventral part of the hypothalamus 

 converge into the commissura tuberis in the caudal part of the 

 postoptic commissure complex. After crossing here in diffuse arrange- 

 ment, they recurve around the tuberculum posterius, where most of 

 them spread and end in the peduncle (tr. hypothalamo-peduncularis) . 

 Others descend in ventral tegmental fascicles of group (4) as tr. hy- 

 pothalamo-tegmentalis. Only the longer fibers of the latter tract are 

 entered on the drawings of the horizontal sections (figs. 25-32, 

 marked tr.hy.teg.{Jf.), or simply (4)). Figure 32 cuts these fibers (^) 

 at the most dorsal level of their arched course through the peduncle; 

 compare their projection on the sagittal plane ('36, fig. 3, po.{4-)). 

 Other similar fibers extend dorsally from both ventral and dorsal 

 parts of the hypothalamus to enter the midbrain without decussation 

 in the chiasma ridge ('42, figs. 22, 23, 39). Some of these decussate in 

 component 2 of the commissure of the tuberculum posterius, but 

 most of them are uncrossed. 



This system of fibers is evidently the main descending pathway 

 from the ventral hypothalamus and neuropil of the chiasma ridge to 

 the motor field of the peduncle and tegmentum. The associated fibers 

 connected with the dorsal part of the hypothalamus probably shoiild 

 be classed with the mamillo-peduncular system, though they are not 

 included in the bundles so designated. This more dorsal system, and 

 perhaps the ventral system also, contain some fibers which are af- 

 ferent to the hypothalamus, though most of them evidently are 

 efferent. Most of the longer fibers of this system which reach the teg- 

 mentum descend in bundles of group (4), and here they are joined 

 by thalamo-tegmental and pedunculo-tegmental fibers ('42, p. 225 

 and fig. 4). The longest fibers enter the f. longitudinalis medialis 

 (figs. 27, 28), and it is uncertain whether these are of hypothalamic 

 or of peduncular origin. 



Most of the hypothalamic fibers of groups (3) and (4) pass into 

 f. tegmentalis profundus, and most of these end in the neuropil 



