300 THE BRAIN OF THE TIGER SALAMANDER 



origin from the dorsal thalamus and similar distribution in the teg- 

 mentum — tr. thalamo-tegmentalis rectus (figs. 15, 31-34, 94, 

 tr.th.teg.r.). This latter tract arises from both dorsal and ventral 

 thalamus, but only the dorsal component of it is under consideration 

 here (fig. 21, tr.th.teg.d.r.). In some Golgi preparations there is evi- 

 dence that axons from the dorsal thalamus may divide, with branches 

 entering both the uncrossed and the crossed thalamo-tegmental 

 tracts ('42, p. 224). Evidently, the crossed and uncrossed tracts are 

 reciprocally related physiologically. 



10. Tractus thalamo-tegmentalis dorsalis cruciatus B. — This is a 

 deep component of the same system as the preceding, receiving 

 nearly all the myelinated fibers of no. 7 of this list. In the chiasma 

 ridge these fibers separate from the others of the group and cross at 

 the dorsal border of the postoptic commissure (marked B in figs. 26 

 and 95). Beyond the decussation they scatter widely, and most of 

 them enter the well-myelinated tegmental fascicles of group (8), 

 within which they may descend as far as the V nerve roots. 



The two crossed tracts to the tegmentum, nos. 9 and 10 of this list, 

 seem to have the same origin and about the same field of distribution, 

 except that one of them {A) terminates in the superficial tegmental 

 neuropil and the other {B) arborizes in the deep neuropil. The 

 physiological properties of these zones of neuropil evidently are 

 different. 



11. Tractus thalamo-tegmentalis ventralis cruciatus (figs. 2C, 17, 

 tr.th.teg.v.c). — These thick fibers (many of them well myelinated) 

 converge into the postoptic commissure from all parts of the ventral 

 thalamus. In this part of their course they are not fasciculated but 

 are scattered among other similar fibers so that their courses could 

 not be followed until they were studied embryologically. They ma- 

 ture early and may be impregnated with reduced silver at stages 

 when few other fibers respond to this treatment. We also have good 

 elective Golgi impregnations of them in later larval stages ('39, pp. 

 98, 120; '396, p. 546; '42, p. 225). 



These fibers cross in the posterodorsal part of the chiasma ridge, 

 mingled with those of other systems. The thickest of the myelinated 

 fibers are crowded together at the dorsal margin of the postoptic com- 

 plex. After crossing, they spread widely in the peduncle and teg- 

 mentum. Most of those from the posterior part of the ventral thala- 

 mus enter ventral tegmental fascicles of group (4), and some of these 

 may descend in the f . longitudinalis medialis. Many fibers from the 



