234 THE BRAIN OF THE TIGER SALAMANDER 



EPITIIALAMUS 



The epithalamus, like all other parts of the sensory zone, combines 

 reception from the periphery with functions of correlation. It has 

 two radically different parts that are separated by the pineal recess 

 and the overlying pineal vesicle. The habenular nuclei comprise the 

 anterior division. The posterior division is the pars intercalaris 

 diencephali, which is relatively larger in urodeles than in higher 

 animals. Because of this enlargement and the small size of the dorsal 

 thalamus, the habenula lies farther forward than usual. The two 

 habenulae are connected by the habenular or superior commissure 

 and the pars intercalaris by the commissura tecti diencephali. 



The pars intercalaris is sharply defined in front, but posteroven- 

 trally it merges with the posterior sector of the dorsal thalamus. 

 There is a narrow subependymal layer of small nerve cells imbedded 

 in dense periventricular neuropil, and externally of this lies a lenti- 

 form mass of crowded cells, mostly small, with some of larger size, 

 which I term the "pretectal nucleus" (figs. ^B, 14, 15, 35, 36, nuc.pt.) 

 without commitment about its homologies in other animals. It is 

 probable that the pulvinar of mammals has been derived from the 

 undifferentiated gray of the posterior part of this field. 



As described in 1942 (pp. 205, 259, 279), the pretectal nucleus is 

 permeated and covered with dense neuropil, which is continuous with 

 that of all surrounding parts — tectum, habenula, and dorsal thala- 

 mus — with fibers passing in both directions. It receives terminals and 

 collaterals from all chiasmatic bundles of the optic tracts (figs. 14, 

 22), tectum (figs. 12, 14, ir.t.pi.), posterior commissure, dorsal thala- 

 mus, habenula, and tr. strio-tectalis (figs. 14, 101, tr.st.pt.). Efferent 

 fibers leave this nucleus for the tectum (figs. 11, 15, tr.pt.tec), 

 thalamus (fig. 15), hypothalamus (figs. 15, 16, 25-36, tr.pt.hy.), and 

 peduncle (tr. thalamo-peduncularis cruciatus and tr. thalamo-pedun- 

 cularis dorsalis superficialis) . Some fibers accompany the fasciculus 

 retroflexus to reach the area ventrolateralis pedunculi (fig. 23). 



In my previous publications the tr. pretecto-hypothalamicus was 

 not identified. Here its course is shown in figures 25-36. Many 

 unmyelinated fibers, accompanied by a few with myelin sheaths, 

 leave the pretectal nucleus (figs. 35, 36) and descend near the surface 

 rostrally and internally of the marginal optic tract. Many of these 

 fibers end in the thalamus (tr. pretecto-thalamicus, tr.pt. th.), and the 

 remainder descend to decussate in the postoptic commissure and 

 spread in the hypothalamus. These fibers and those here termed tr. 



