MIDBEAIN AND THALAMUS OF NECTURUS 223 



lies caudad of this sulcus and thus reaches forward to the fovea 

 isthmi. 



In older larvae, as the mesencephalic flexure is straightened 

 out, this sulcus comes to be directed caudad as well as dorsad 

 from the fovea isthmi and it becomes very deep and narrow. 

 Its relations to the eminentia subcerebellaris tegmenti in a 38 

 mm. larva are shown in figure 2 of my paper ('14 a, p. 391). 

 Its rostral end reaches the fovea isthmi. In all half grown 

 larvae the ventral end of the strong external fissura isthmi or 

 isthmic constriction is bent sharply rostrad to meet the ventral 

 surface at a point a short distance caudad of the superficial origin 

 of the III nerve, i.e., at about the level of the fovea isthmi. 

 The external fissura isthmi, therefore, takes a course which 

 closely parallels the ventricular sulcus isthmi, the two con- 

 strictions marking a thin line in the wall of the brain tube which 

 separates the thicker cerebral peduncle in front from the still 

 more massive eminentia subcerebellaris tegmenti behind, the 

 latter appearing as a distinct eminence on both the external 

 and the ventricular surfaces of the brain (see figs. 2 and 3 of my 

 paper, '14 a). 



In adult Amblystoma the relations of the isthmic fissure and 

 sulcus are the same, though not so sharply defined as in the 

 larvae, and the eminentia subcerebellaris tegmenti is much 

 elongated. In adult NecturUs the sulcus isthmi is a distinct, 

 though shallow, ependymal groove (figs. 63, 64, s.is.), extend- 

 ing dorso-caudad from the fovea isthmi, and the eminentia 

 subcerebellaris tegmenti is still more elongated. 



From these relations it is clear that the eminentia subcere- 

 bellaris tegmenti lies in the rhombencephalon and the fovea 

 isthmi marks, as Stieda supposed, the caudal boundary of the 

 pedunculus cerebri on the ventral surface. It follows that in 

 Necturus the floor of the midbrain is limited to the short dis- 

 tance between the tuberculum posterius and the fovea isthmi. 



The complex decussations in the floor of the midbrain are 

 commonly referred to by comparative neurologists as the an- 

 sulate commissure, but since this term as usually apphed in- 

 cludes also the tegmental decussations below the fovea isthmi 



