THE HABENULA AND ITS CONNECTIONS 257 



and of the diencephalon are massive and direct ('22«, fig. 8), and the 

 habenular tracts are not fasciculated to form a stria medullaris. With 

 further evagination of the hemisphere in amphibians and the appear- 

 ance of a deep stem-hemisphere fissure, the habenuhir connections of 

 the palHal parts of the hemisphere must turn downward to pass under 

 the floor of this fissure. Here they are joined by other habenular 

 tracts to form the stria medullaris thalami. These tracts of Amblys- 

 toma were described in 1910 (and further details, '27, p. 284 and figs. 

 15-18; '396, p. 538), and now some corrections and additions can be 

 contributed. Compare also my description of the habenular connec- 

 tions of Necturus ('336, pp. 204-14). 



The components of the stria which have been identified in Amblys- 

 toma are shown in figure 20, where they are projected upon the 

 median plane. The courses of four of the more lateral components are 

 shown as projected upon the lateral aspect of the brain in figure 85. 

 Their arrangement as seen in horizontal sections is shown in figures 

 25-36 and in sagittal sections in figures 74-78. Some components are 

 electively impregnated in our Golgi preparations, and these speci- 

 mens have been especially useful in demonstration of the truly com- 

 missural connections of several of the tracts listed below. Compo- 

 nents 3, 4, 5, and 6 are known to decussate in com. superior telen- 

 cephali and component 8 in com. pallii posterior. On the figures the 

 components of the stria are numbered as in the following list: 



1. Tractus olfacto-habenularis medialis. 



2. Tractus olfacto-habenularis lateralis. 



Most of the fibers of these two tracts are axons of cells of the pre- 

 optic nucleus, ascending, respectively, medially and laterally of the 

 basal forebrain bundles (figs. 25-30, 74-77; '27, fig. 16; '42, figs. 18, 

 24), but some of them are collateral branches of axons of the medial 

 forebrain bundle ('396, p. 538). This is doubtless true in Necturus 

 also, though I was not able to demonstrate it ('336, p. 208). In the 

 passage last cited, references are given to descriptions of this connec- 

 tion in other urodeles. The fibers of these tracts are assembled from 

 all parts of the preoptic nucleus, ascending on both sides of the sulcus 

 preopticus. The fibers of the medial tract pass through the gray of the 

 nucleus of this tract, with many terminals and collaterals, and here 

 the tract receives accessions from the nucleus. More dorsally both 

 tracts have collateral connections with the eminentia thalami and 

 neuropil of Bellonci. They form the most posterior component of the 

 stria medullaris, and within the habenular nuclei they spread out in 



