250 THE BRAIN OF THE TIGER SALAMANDER 



the dorsal and the ventral thalamus. I have regarded it as part of the 

 dorsal thalamus, though its morphological status is uncertain. This 

 nucleus was not recognized by Bellonci ('88), hut he did describe in 

 the frog a peculiar field of very dense neuropil related with it, which 

 I term the "neuropil of Bellonci" in preference to his name, ''nucleus 

 anterior superior corporis geniculati thalami." This nucleus and as- 

 sociated neuropil I first described in Necturus ('17, p. 243) under the 

 name of "pars optica thalami," but this term also is inappropriate 

 and has been discarded. In Necturus and Ambly stoma this peculiar 

 neuropil receives terminals and collaterals from the optic tract, stria 

 medullaris, tr. cortico-thalamicus medialis, the anterior tectal fascic- 

 ulus (p. 297), and other sources. Axons of the cells of the nucleus go 

 to the habenula and to the ventral thalamus and peduncle. This 

 neuropil has received much study ('25, p. 454; '27, p. 295; '33&, 

 p. 216; '34, p. 107), but, as recently remarked ('42, p. 279), many 

 details of its structure and connections are still obscure. The probable 

 equivalent of this nucleus has been found in all major groups of 

 vertebrates by Addens ('46 and papers there cited). 



6. Habenula. — The general structure and chief connections of the 

 amphibian habenular nuclei are well known and have been sum- 

 marized by Ariens Kappers, Huber, and Crosby ('36). There are two 

 habenular nuclei on each side, different in position from that of mam- 

 mals but with similar fibrous connections. They are arranged one 

 above the other at the anterodorsal border of the diencephalon, dor- 

 sally of the eminentia thalami. The taenia thalami, or line of attach- 

 ment of the membranous dorsal sac, extends from the eminentia 

 thalami along the anterior border of both nuclei and backward along 

 the dorsolateral border of the dorsal nucleus as far as the habenular 

 commissure. Posteriorly of this commissure the roof is again mem- 

 branous as far as the recessus pinealis. The dorsal and ventral nuclei 

 are separated on the ventricular surface by a shallow sulcus intra- 

 habenularis and bounded posteriorly by the sulcus posthabenularis, 

 which is the surviving dorsal end of the sulcus diencephalicus ven- 

 tralis of Rudebeck ('45). Below the ventral nucleus is the deep sulcus 

 subhabenularis. This is essentially the embryonic arrangement, in 

 contrast with that of adult man, where the great enlargement of the 

 dorsal thalamus and reduction of the membranous dorsal sac and 

 paraphysis result in great changes in the relative positions of parts. 



The ventral and dorsal habenular nuclei of Amblystoma are prob- 

 ably the equivalents, respectively, of the lateral and medial nuclei of 



