DIENCEPHALON 237 



tricular eminence inclosed by the dorsal and middle thalamic sulci 

 (fig. 2, s.d. and s.m.). It is the precursor of several of the sensory 

 nuclei of the mammalian thalamus, though there is no visible local 

 differentiation of its gray substance corresponding to these nuclei. 

 This homology is clearly established by the location of these cells and 

 their fibrous connections, which conform, as far as they are present, 

 with the mammalian pattern (figs. 14, 15). This area receives ter- 

 minals and collaterals of the optic tract, terminals of the spinal and 

 bulbar lemniscus, strong tecto-thalamic tracts (precursors of the 

 brachia of the superior and inferior colliculi), some fibers from the 

 habenula, and probably also fibers from the hypothalamus by way of 

 the postoptic commissure systems and from the cerebral hemisphere 

 accompanying the tr. strio-pretectalis. These fibers all enter a com- 

 mon pool of neuropil, which is most dense in the intermediate alba 

 but which also pervades the entire structure. 



From this thalamic pool, efferent fibers go out to all surrounding 

 parts of the brain, some in the diffuse neuropil and some as well- 

 fasciculated tracts. An extensive and complicated system of efferents 

 discharges into the underlying motor zone of the same and the op- 

 posite side, including direct and crossed tracts to the ventral thala- 

 mus, hypothalamus, cerebral peduncle, and isthmic and bulbar teg- 

 mentum. The more important of these are shown in figure 15. A large 

 tract containing some thick myelinated fibers (tr.th.p.) joins similar 

 fibers from the pretectal nucleus and tectum (fig. 18, tr.t.p.c), par- 

 tially decussates in the commissure of the tuberculum posterius, and 

 spreads through the alba of the peduncle on both sides, as illustrated 

 in figures 30-36 {tr.th.p.c. and tr.t.p.c. 1.). Parallel with these deeper 

 fibers there is a very large superficial uncrossed tract from the dorsal 

 thalamus and pretectal nucleus to the peduncle (not shown on the 

 figures) similar to that described in Necturus as tr. thalamo-pedun- 

 cularis dorsalis superficiahs ('17, p. 264). Other similar fibers are dis- 

 persed in the neuropil, some of which go directly to the ventral part 

 of the thalamus — tr. dorsoventralis thalami (shown but not labeled 

 in figs. 15 and 16; see '17, p. 266). A small number of fibers go to the 

 habenula (tr.th.hab.), and a larger number to the tectum in the 

 brachia of the superior and inferior colliculus {br.coL). The large tr. 

 thalamo-tegmentalis rectus includes deep and superficial fibers to the 

 dorsal and isthmic tegmentum (figs. 32-34, 94, tr.th.teg.r.) ; for those 

 which decussate in the postoptic commissure see chapter xxi and 

 '42, p. 223. 



