THE COMMISSURES 299 



7. Tractus thalamo-hypothalamicus et peduncnlaris cruciatus 

 {tr.ih.h.p.c.). — In the earlier descriptions of both Aniblystoma and 

 Necturus this name was given to a large collection of unmyelinated 

 and lightly myelinated fibers which descends in disj^ersed arrange- 

 ment from the dorsal thalamus to the postoptic commissure. Their 

 distribution beyond the decussation could not be clearly followed, 

 and some of those descriptions now require correction. As elsewhere 

 pointed out ('42, p. 223), this name should now be discarded because 

 at least three quite distinct tracts are represented here and relatively 

 few of these fibers have any connection with the peduncle. The three 

 tracts represented in this complex (nos. 8, 9, and 10) have a common 

 origin in the dorsal thalamus, chiefly its middle sector, and, after 

 crossing, take widely different courses. Their decussation is posterior 

 to that of the tectal components of the commissure in a band which 

 is narrow dorsally and spreads ventrally through a wide area of the 

 neuropil of the chiasma ridge (fig. 2C, tr.th.h.d.c). 



8. Tractus thalamo-hypothalamicus dorsalis cruciatus (tr.th.h.d.c). — 

 These fibers, most of which are unmyelinated, cross in the middle 

 region of the chiasma ridge (figs. 2C, 15, 25) and then spread in the 

 hypothalamus. In figures 27-33, 95, 102, and 103 the symbol 

 tr.th.h.d.c. refers to the mixture of fibers of tracts 8, 9, and 10 in their 

 descending course from the thalamus to the commissure. The fibers 

 of nos. 9 and 10 cross dorsally of those of no. 8, though there is min- 

 gling of the fibers of the three tracts with one another and with those 

 of surrounding decussations. 



The remaining fibers of this complex, after crossing, are distributed 

 to the tegmentum in two tracts which take parallel courses, one su- 

 perficially, the other at the border of the gray. These are designated 

 components A and B, respectively. In figures 15 and 21 the com- 

 ponents A and B are not separately designated. 



9. Tractus thalamo-tegmentalis dorsalis cruciatus A {tr.th.teg.d.c.A.). 

 — This tract was first described in the early feeding larva ('39, p. 116) 

 and later in the adult ('42, p. 224). After decussation, these unmyeli- 

 nated fibers ascend from the chiasma ridge parallel with the course of 

 the descending uncrossed limb of this commissure and more super- 

 ficially. Their further course is shown in figures 26-34, here marked 

 A . In the dorsal tegmentum they turn spinal ward along the ventro- 

 lateral border of the tectum. Here they lie close to the pial surface 

 and immediately ventrally of the lateral optic tract (fig. 94, A). In 

 this part of their course they join an uncrossed tract with similar 



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