260 C. JUDSON HERRICK 



Unmyelinated fibers of this system are especially numerous 

 in the more caudal part of the postoptic commissure and these 

 fibers turn caudad into the motor tegmentum of the cerebral 

 peduncle in a very superficial position. 



The fibers of this system which arise from the pars dorsalis 

 thalami and after decussation end in the hypothalamus {tractus 

 thalamo-hypothalamicus cruciatus) reach both the dorsal and 

 the ventral parts of the hypothalamus. Their area of distribu- 

 tion is more medial than that of the hypothalamic fibers of the 

 tractus tecto-thalamicus et hypothalamicus cruciatus. None 

 of them extend backward into the caudal or free part of the 

 pars ventralis hypothalami, but they reach all dorso-ventral 

 levels close behind the postoptic commissure from the extreme 

 ventral surface to the level of the tuberculum posterius. 



The fibers of the tractus thalamo-hypothalamicus cruciatus 

 decussate in the more caudal part of the postoptic commissure. 

 Here the texture of the commissural complex is much more open 

 than farther forward, where the decussating fibers cross in thick 

 masses of straight fibers with no considerable amount of neu- 

 ropil between them. In this more caudal part of the commfs- 

 sure Golgi sections show that the decussating fibers run singly, 

 not in fascicles, they are contorted but not varicose, and they 

 are embedded in an open neuropil formed in part by dendrites 

 from all adjacent parts of the brain (figs. 20, 27, 28). Some of 

 these dendrites come forward from the ventral part of the hypo- 

 thalamus, some arise from neurons of the dorsal part of the 

 hypothalamus, and some come even from the pars ventralis 

 thalami farther dorsally. The nucleus of the tractus pallii 

 lies embedded in the caudal part of this neuropil (figs. 5, 6. 

 67, tr.pal. and nuc.tr. pal.). 



IJf.. Tractus tecto-thalamicus rectus 



From the rostral part of the tectum mesencephali, and per- 

 haps also from its entire extent, a diffuse collection of myelinated 

 and unmyelinated fibers passes between the tectum and the 

 thalamus. These fibers lie at an intermediate level in the stra- 

 tum album deeper than the thalamic fibers of the acoustico- 



