298 THE BRAIN OF THE TIGER SALAMANDER 



where they are marked tr.t.th.h.c.p. before their decussation and (8) 



beyond the crossing. 



There are two strong systems of crossed tecto-peduncular and 

 tecto-tegmental fibers, both of which are drawn in figure 12. The 

 system just described descends chiefly from the nonoptic part of the 

 tectum {tr.t.th.h.c.p.) and, after crossing, spreads in the peduncle by 

 way of tegmental fascicles (6) and throughout the tegmentum by way 

 of fascicles (8). The second system is tr. tecto-peduncularis cruciatus 

 (tr.t.p.c), which arises in the optic part of the tectum, crosses in the 

 commissure of the tuberculum posterius, and then spreads out in the 

 peduncle. This well-myelinated tract is accompanied by similar fibers 

 from the pretectal nucleus and dorsal thalamus. These two tecto- 

 peduncular systems evidently have quite different physiological sig- 

 nificance. 



Attempts to analyze the postoptic components arising in the 

 thalamus were unsuccessful until the sequence of development of 

 these fibers was revealed by embryological studies. These findings 

 were then confirmed by elective Golgi impregnations of older larvae 

 and adults. Some errors in the earlier descriptions have been cor- 

 rected, and now it is possible to give a fairly complete account of 

 both the crossed and the uncrossed fibers which diverge from the 

 thalamus. Since the direct and crossed fibers are evidently intimately 

 related physiologically, both series are included in the following de- 

 scription. Efferent fibers from the thalamus are arranged in two 

 sharply contrasted series, which arise, respectively, from the dorsal 

 thalamus and the ventral thalamus. 



The efferent series from the dorsal thalanuis includes uncrossed 

 fibers to the tectum, habenula, cerebral hemisphere, ventral thala- 

 mus, hypothalamus, and peduncle which need not be further con- 

 sidered here; but some of the other uncrossed tracts, which evidently 

 are in reciprocal physiological relation with the crossed tracts, should 

 be specifically mentioned. Decussating fibers from the dorsal thala- 

 mus are in two groups. The first includes thick myelinated fibers of 

 tr. thalamo-peduncularis cruciatus (tr.th.p.c), which, as already 

 mentioned, joins tr. tecto-peduncularis cruciatus {tr.t.p.c.) to decus- 

 sate in the commissure of the tuberculum posterius as described un- 

 der that caption below. The second group is a much larger number of 

 thin unmyelinated or lightly myelinated fibers which decussate in the 

 postoptic commissure and will next be described. 



