THE HABENULA AND ITS CONNECTIONS 251 



higher forms, but the details of their connections are not exactly 

 comparable. Both nuclei receive terminals of the stria medullaris 

 thalami (apparently all components of it), and the chief efferent path 

 of both nuclei is the tr. habenulo-interpeduncularis, which is the 

 largest component of the fasciculus retroflexus (of Meynert). The 

 dorsal nucleus is connected with the tectum and pretectal nucleus by 

 fibers running in both directions, and the ventral nucleus is similarly 

 connected with the dorsal thalamus, the ventral border of the tectum, 

 and the dorsal tegmentum. 



In the ventral habenular nucleus the nerve cells are relatively few 

 in the usual arrangement at the ventricular side. The alba is almost 

 completely filled by the massive stria medullaris, internally of which 

 is neuropil, which invades the gray substance. Most of the cell bodies 

 are widely separated within this dense neuropil. The larger dorsal 

 nucleus has very numerous small cells, densely crowded on all sides 

 except dorsally of the commissure. This layer of cell bodies incloses a 

 central core of dense neuropil, which connects with the stria medul- 

 laris below and the habenular commissure above. The thick contorted 

 dendrites ramify in the central neuropil (fig. 73; '42, figs. 77, 79). 



Primitively, these nuclei had direct connection with the parietal 

 eye, as is the case in some still living species. It is possible that in 

 some early ancestor of the vertebrates, now extinct, the dorsal parie- 

 tal eye was better developed than the lateral eyes and that for this 

 reason the primordial olfacto-visual correlation was made in the 

 epithalamus rather than in the tectum of the midbrain. 



The habenular system is one of the most conservative parts of the 

 vertebrate brain. Edinger's statement ('11, p. 370) that this is per- 

 haps the only part of the brain the organization and connections of 

 which show no alterations during the whole course of vertebrate 

 phylogeny requires some qualification; yet, as we pass from cyclo- 

 stomes to man, with revolutionary changes in all surrounding parts, 

 the chief habenular connections show a surprising uniformity. 



In urodeles afferent fibers enter the habenula from all parts of the 

 cerebral hemisphere, except perhaps the olfactory bulb (some au- 

 thorities would not make this exception) and also from the preoptic 

 nucleus, thalamus, and tectum. The largest of these tracts come from 

 areas which are under the strongest olfactory influence, and the 

 habenular complex is generally regarded as primarily concerned with 

 olfactory adjustments. That this is not its only function is evident 

 from the fact that in anosmic animals, like some birds and cetaceans, 



