MIDBRAIN AND THALAMUS OF NECTURUS 259 



with the pars dorsahs hypothalami and the nucleus of the tuber- 

 culum posterius. True commissural fibers from the tectum of 

 one side to that of the opposite side may be present in both parts 

 of this system. 



13, Tractus thalamo-hypothalamicus et peduncularis cruciatus 



This complex system of fibers forms a second very important 

 component of the postoptic commissure (figs. 1 to 7, 20, 25, 27, 

 28, 55, 57, 58, 62, 67, 68 tr.th.h.p.c. and tr.th.h.p.c.x.). Its 

 fibers, which are both myelinated and unmyelinated, arise from 

 the pars dorsalis thalami and pass ventralward laterally of the 

 lateral forebrain bundle. They leave the thalamus in company 

 with the unmyehnated tractus thalamo-frontalis anterior, but 

 in much more diffuse formation (fig. 55). The unmyelinated 

 fibers at their decussation are rather widely scattered through 

 the chiasma ridge. The myelinated fibers decussate in the 

 dorsal part of the postoptic commissure complex dorso-caudally 

 of the decussation of the tractus tecto-thalamicus et hypothal- 

 amicus cruciatus posterior (fig. 62), and after their crossing 

 these fibers ascend at a deeper level than the descending fibers, 

 penetrating the lateral forebrain bundle and then turning 

 abruptly caudad into the pars ventralis thalami and the cere- 

 bral peduncle dorsally of the latter bundle. 



Some fibers of this system which are more heavily myelinated 

 than any others decussate in the caudal part of the postoptic 

 commissure and then form a compact fascicle which ascends in 

 the deepest level of the stratum album (figs. 4 to 7, 67, 68, tr.th. 

 h.p.c.x.) and turns caudad along the ventral border of the lateral 

 forebrain bundle. The fibers of this crossed thalamo-pedun- 

 cular tract (tractus thalamo-peduncularis cruciatus), mingled with 

 the longer fibers of the lateral forebrain bundle, form the most 

 dorsal fasciculi of the motor tegmentum. In larval Amblystoma 

 I have followed ('14 a, figs. 4 to 9, tr.th.b.) the tractus thalamo- 

 peduncularis cruciatus (there called tractus thalamo-bulbaris) as 

 far back as the superficial origin of the V cranial nerve. 



