THE COMMISSURES 297 



it leaves the pretectal nucleus is accompanied by a large tr. pretecto- 

 thalamicus (p. 234 and figs. 35, 36). 



4. Tr actus tecto-ihalamicu.s ei hypothalamicus cruciatus posterior 

 (fig. 12, tr.t.th.h.c.p). — This, like no. 1, is a mixed fascicle which has 

 been analyzed. Both these names may now be discarded in favor of 

 shorter terms — anterior and posterior tectal fascicles. The posterior 

 fascicle arises chiefly from the nonoptic nucleus posterior tecti and 

 the adjoining ventrolateral margin of the optic tectum, the latter 

 region in Necturus receiving few terminals of the optic tract ('41a, 

 p. 516). This fascicle is probably activated primarily by lemniscus, 

 rather than optic, fibers or by a combination of the two. The tracts 

 of which it is composed have collateral connections with the genicu- 

 late neuropil and ventral thalamus both before and after crossing. 

 These fibers descend from the tectum parallel with those of the lateral 

 optic tract and internally of them. So far as known they have a com- 

 mon origin in the tectum, but, after crossing, they take widely 

 divergent courses. The tracts, consequently, are named according to 

 their terminal distribution. The most important components are the 

 following, nos. 5 and 6. 



5. Tractus tecto-hypothal amicus posterior. — These are finer fibers 

 which terminate in the postoptic neuropil and neighboring regions of 

 the hypothalamus. They are more clearly seen in Necturus ('41a, 

 p. 516) than in Ambly stoma. 



6. Tractus tecto-tegmentalis cruciatus (fig. 12, tr.t.teg.c). — The 

 course and distribution of the thicker fibers of the posterior tectal 

 fascicle were not clarified until they were identified in larval stages, 

 in which they were electively impregnated because they mature 

 precociously. These were first recognized in early feeding larvae ('39, 

 p. 106) and later in adult Necturus ('41a, p. 516) and Amblystoma 

 ('42, p. 222). Some erroneous descriptions of these fibers in my 

 earlier papers have been corrected ('39, p. 110). 



The more heavily myelinated fibers of this tract decussate in the 

 dorsal part of the chiasma ridge (figs. 2C, 12, 16, 25, tr.t.th.h.c.p.), 

 and after crossing they enter tegmental fascicles of groups (8) and 

 (6), and in smaller numbers they are dispersed in other fascicles. The 

 dispersed fibers spread in the peduncle. Those which enter fascicles 

 numbered (8) take a longer and more dorsal course, distributing to 

 the dorsal, isthmic, and trigeminal tegmentum, some of them ex- 

 tending as far as the level of the V nerve roots. The entire course of 

 these fibers can be followed in the horizontal sections, figures 25-35, 



