18(! THE BRAIN OF THE TIGER SALAMANDER 



nianinialian neurology. Component 4, I lie superior visceral nucleus, 

 has been described above (p. 18'-2). 



WHITE SUBSTANCE 



In the white substance of the isthmus the most conspicuous fea- 

 tures are the numbered tegmental fascicles described in chapter xx. 

 Externally of these fascicles are the massive teclo-bulbar and tecto- 

 spinal tracts and, suj)erficially close to the pial surface, a layer of 

 line fibers, most of which are unmyelinated. The latter includes tr. 

 thalamo-tegmentalis rectus from dorsal and ventral thalamus (fig. 

 94; '36, }). .'UO) and, more dorsally, similar fibers from the dorsal 

 thalamus, which decussate in the postoptic commissure — tr. thala- 

 mo-tegmentalis dorsalis cruciatus (p. '-29!); '39, pp. 95, 116). 



Mingled with these fibers and more abundantly at deeper levels 

 are fibers of tr. tecto-peduncularis and tecto-tegmentalis (figs. 18, 

 22, 24; '42, p. 267 and figs. 14, 30; Necturus, '17, figs. 9-14, 32, 33, 

 tr.t.ped.p.). Short fibers from the entire tectum and dorsal tegmentum 

 to the isthmic tegmentum are dispersed in both white and gray sub- 

 stance, and a large number of the longer fibers are assembled in dor- 

 sal tegmental fascicles of group (7) accompanying tr. tecto-bulbaris 

 rectus (p. 283). 



Figure 21 shows only the more prominent att'erent connections; 

 there are many others. In the aggregate these include fibers from the 

 motor zone above and below the isthmus, collaterals of the lemniscus 

 sj^stems in the isthmic neuropil, terminals of the ascending visceral- 

 gustatory tract, cerebello-tegmental fibers, short fibers from the 

 overlying dorsal tegmentum and tectum, longer fibers from the tec- 

 tum which decussate in the postoptic and ventral commissures, di- 

 rect fibers from the dorsal and ventral thalamus, strong tracts from 

 the dorsal and ventral thalamus which decussate in the postoptic 

 commissure, fibers from both cerebral hemispheres by way of the 

 lateral forebrain bundle and tr. olfacto-peduncularis, and strong 

 tracts from the hypothalamus, crossed and uncrossed. To this list one 

 might add the fasciculus retroflexus, passing from the habenula to the 

 interpeduncular nucleus and, through the latter, acting upon the 

 isthmic tegmentum. 



Most of the efferent fibers descend in the f. tegmentalis profundus 

 (p. 286) and the ventral tegmental fascicles. Some are dispersed in 

 the neuropil accompanying tr. interpedunculo-bulbaris dorsalis, and 

 these are regarded as jn-ecursors of the bulbar part of the f. longitu- 

 dinalis dorsalis of Schlitz (p. 208). The relativelj^ short fibers which 



