660 C. JUDSON HERRICK AND JE ANNETTE B. OBENCHAIN 



in the commissura superior. This suggests a very intmiate rela- 

 tion between this region and the habenula. 



Johnston ('12, p. 345) describes two vertical ridges under the 

 habenula, which he says are related respectively to the stria med- 

 ullaris and the tractus habenulo-peduncularis (fasciculus retro- 

 flexus of Meynert) and are separated by a vertical sulcus termed 

 'sulcus medius.' The latter he compares with the amphibian sul- 

 cus medius of Herrick. In Ichthyomyzon his sulcus medius (p . 349, 

 and fig. 6) evidently corresponds with our sulcus thalamicus 3. His 

 eminentia thalami is divided by a small sulcus which he regarded 

 as an artefact (p. 377) into two vertical ridges. This sulcus is 

 our sulcus thalamicus 2. The anterior part of Johnston's emi- 

 nentia thalami corresponds to our eminence of the same name. 

 Its posterior part and his ridge related to the tractus habenulo- 

 peduncularis form our subhabenular lobe, these relations being 

 confirmed by a study of the left side of our model, where the 

 tracts related to the habenula are smaller and thus permit a 

 better analysis of the cellular masses (cf. p. 647). 



The eminentia thalami was first defined by Herrick ('10, p. 

 419) in the Amphibia. Its functional connections are still im- 

 perfectly understood, but dentrites of its neurones are known to 

 be related to fibers of the adjacent stria medullaris and the neu- 

 rites of these cells in Amphibia pass backward into the pars ven- 

 tralis thalami. For these reasons it was regarded as functional- 

 ly related to the pars ventralis thalami, from which, however, it 

 is separated in the urodele amphibians by a well defined vertical 

 sulcus, and from which it differs greatly in internal structure. 

 The corresponding structure in Ichthyomyzon appears to be di- 

 vided into two parts, a ventral and a dorsal, both of which are 

 differently relatecl to the other parts of the diencephalon than in 

 Amphibia by reason of the peculiar relations of the stria medul- 

 laris in cyclostomes. 



The components which make up the. stria medullaris of higher 

 brains appear to be separated in the petromyzonts into two imper- 

 fectly distinct groups, a ventral and a dorsal (Johnston '12, p. 

 358). The ventral group of fibers arise chiefly from the nucleus 

 preopticus, nucleus olfactorius medialis and lobus olfactorius and 



