THE HABENULA AND ITS CONNECTIONS 249 



toma the posterior lobe of the inferior or "proper" nucleus preopti- 

 ons develops posteriorly of the sulcus intraencephalicus anterior; and, 

 more dorsally, Rudebeck's pars superior of this nucleus is clearly the 

 primordium of my nucleus of the olfacto-habenular tract, including, 

 perhaps, also the posterior end of the amygdala. In any case the two 

 nuclei last mentioned are very intimately related. 



4. Eminentia thalami. — This name was given ('10, p. 419) to a 

 prominent ventricular eminence lying behind the interventricular 

 foramen between the anterior commissure ridge and the habenula, 

 which has been variously interpreted by authors. It is appropriately 

 named "nucleus commissurae hippocampi" by Addens ('46). I have 

 regarded it as the anterior end of the ventral thalamus, differentiated 

 as bed-nucleus of several large tracts which converge here. It is well 

 defined in early swimming stages (area 7a) , when the first fibers of the 

 stria medullaris are visible ('38, p. 213 and figs. 18, 19) ; and in early 

 feeding stages, with good development of stria medullaris and hippo- 

 campal commissure, it has acquired essentially adult form ('386, 

 p. 401, figs. 1, 2, 6). It is derived from the middle of the di-telence- 

 phalic ridge of Rudebeck's ('45) pictures of the embryonic brain. 



This eminence receives collaterals from the stria medullaris and in 

 larger numbers from the fibers of the hippocampal commissure (figs. 

 31, 32, 71, 76; '27, p. 294; '35a, p. 250). As the latter fibers swing 

 downward behind the interventricular foramen toward their crossing 

 in the anterior commissure ridge, they are accompanied by similar 

 fibers which pass from the primordium hippocampi to the ventral 

 thalamus — tr. cortico-thalamicus medialis (figs. 31, 32, 33, 72, 75). 

 Like the fibers of the hippocampal commissure, these are mostly 

 unmyelinated, with a few thick, well-myelinated fibers scattered 

 among them. Many of them end in the eminentia thalami, and others 

 continue into the anterior part of the ventral thalamus. This tract is 

 probably the precursor of the mammalian column of the fornix, 

 though here none of its fibers have been seen to reach the hypothala- 

 mus. 



Numberless short, thin, unmyelinated axons descend from the 

 small cells of this eminence to spread in the adjoining ventral thala- 

 mus ('21a, figs. 26-28; '27, p. 269), and thicker axons enter the com- 

 plex crossed and uncrossed system of tr. thalamo-tegmentalis ven- 

 tralis ('39, p. 120 and fig. 2; '396, p. 546; '42, p. 225). 



5. Nucleus of Bellonci. — This name is applied to a group of cells 

 which form a low ventricular eminence between the anterior parts of 



