296 THE BRAIN OF THE TIGER SALAMANDER 



This generalized arrangement as seen in urodeles is probably 

 primitive and may be taken as the point of departure in the study of 

 the postoptic systems of more specialized brains of both fishes and 

 higher vertebrates. These commissures have been analyzed in Nec- 

 turus ('41a, p, 513), where they are still more generalized. Our pres- 

 ent knowledge of these systems of Amblystoma was summarized on 

 pages 219-28 of the paper of 1942, with diagrams illustrating the con- 

 nections of the principal tracts. All known components are assembled 

 in the following list. Here some of the earlier names of tracts have 

 been replaced by more accurate terms; some others are retained, 

 though now known to be inappropriate or inadequate. In this list 

 there are included, first, the groups of fibers which descend to the 

 chiasma ridge from the tectum and pretectal nucleus, followed by 

 those descending from the thalamus, next, the systems arising in the 

 hypothalamus, and, finally, a heterogenous group with hypothalamic 

 connections. For additional details about some of these tracts in 

 preceding chapters consult the Index. 



1. Tr actus tecto-thalamicus et hypothalamicus cruciatus anterior (fig. 

 12, tr.t.th.h.c.a.). — This anterior tectal fasciculus is a mixture of 

 myelinated and unmyelinated fibers from the dorsomedial part of the 

 tectum opticum and the pretectal nucleus which descend across the 

 thalamus in company with the more anterior fascicles of the optic 

 tract. After partial crossing in the antero ventral part of the chiasma 

 ridge, its fibers spread in the neuropil of the chiasma ridge and 

 hypothalamus. Some of them may reach beyond this region. This 

 cumbersome name was applied in my earlier papers to a mixed 

 fascicle which had not been analyzed. It can now be replaced by the 

 names of the several tracts of which it is composed. Some of these 

 are uncrossed (notably tr. pretecto-thalamicus) , and some fibers of 

 two of them decussate in the chiasma ridge, nos. 2 and 3 below. The 

 arrangement of these components as seen in horizontal sections is 

 shown in figures 25-36. 



2. Tractus tecto-hypothalamicus anterior (tr.t.hy.a.). — This is the 

 tectal component of the preceding fasciculus; it is evidently an optic 

 pathway to the hypothalamus (p. 224). It passes through the pretec- 

 tal nucleus and is accompanied by tecto-pretectal fibers. It probably 

 is physiologically related with no. 3. 



3. Tractus pretecto-hypothalamicus (fig. 15, tr.ptJiy.). — This tract as 



