THE HABENULA AND ITS CONNECTIONS ^55 



out in the preceding description of the eminentia thalami, marks the 

 beginning of the differentiation of the mammahan columna fornicis. 

 Some of these fil)ers are collaterals of those of the com. hippocampi 

 and tr. cortico-habenularis medial is. In Ambly stoma none of these 

 fibers have been followed as far as the dorsal (mamillary) part of the 

 hypothalamus, so that the homology with the mammalian postcom- 

 missural fornix is incomplete. In other urodeles it has been described 

 as reaching the hypothalamus (Rothig, '24, pp. 10, 15; Salamandra, 

 Kreht, '30, p. 252). The precommissural fornix is an ancient system, 

 being well developed in many fishes. In Ambly stoma it is large and of 

 typical pattern. These fibers descend from the hippocampal forma- 

 tion rostrally of the foramen to the medial forebrain bundle (figs. 98, 

 99), within which they descend for an undetermined distance. 



' The transverse sections described in 1927 revealed clearly the rela- 

 tions of the precommissural fornix ('27, p. 311), but the postcom- 

 missural connections were obscure. In Necturus these connections 

 are perfectly clear, and the morphological relations of precommis- 

 sural and postcommissural fornix fibers have been discussed ('336, 

 p. 188). Amblystoma resembles Necturus. Accompanying the un- 

 myelinated fibers of the hippocampal commissure there are similar 

 fibers of tr. cortico-habenularis medialis and tr. cortico-thalamicus 

 medialis, and among these are a few myelinated fibers. In the hori- 

 zontal Cajal sections (figs. 28-32) the very numerous thin, fibers of 

 this complex are not impregnated, but the thick axons are deeply 

 stained. Few of the thick fibers decussate in the hippocampal com- 

 missure (figs. 28, 29, 30). Many of them ascend in the stria medul- 

 laris (figs. 32, 33, 34, tr.c.h.m.), and most of the others pass backward 

 and downward through the eminentia thalami into the ventral 

 thalamus and (probably) the preoptic nucleus (figs. 31, 32, tr.c.th.ni., 

 72, 75). These thick fibers and the much more numerous thin fibers 

 by which they are accompanied I have termed "tr. cortico-thalami- 

 cus medialis." An extension of this tract into the hypothalamus 

 would give the connection described by Kreht ('30) in Salamandra 

 and by Loo ('31, p. 84) as tr. cortico-hypothalamicus in the opossum. 

 The mammalian tr. cortico-mamillaris is a further addition to this 

 system. 



STRIA TERMINALIS 



In mammals the ventromedial septal field is connected with the 

 ventrolateral amygdala by fibers, passing probably in both direc- 

 tions, some ventrallv of the basal forebrain bundles (diagonal band 



