644 C. JUDSON HERRICK AND JEANNETTE B. OBENCHAIN 



in our preparations consists of large pale multipolar cells scattered 

 between the ventricular grey and the outer surface of the brain 

 immediately rostrally of the emerging root fibers. These neu- 

 rones closely resemble in form and position those of the nucleus 

 ruber described by de Lange ('12) in a number of lower verte- 

 brates; but Johnston ('02, p. 12) has traced their neurites directly 

 into the III nerve. The dorsal and ventral nuclei of Tretjakoff 

 are not clearly distinguishable in our preparations. Their cells 

 are smaller than those of the lateral nucleus and are closely 

 packed around the fourth pair of giant cells of Mtiller. The root 

 fibers from these cells decussate in large numbers and form a 

 ridge in the floor of the ventricle which forms the upper boundary 

 of a small medial recess, the recessus oculomotorius (fig. 3, r.III.). 

 The decussation of the III root fibers is separated by the entire 

 width of this recess from the underlying commissura ansulata. 



Sulcus medius. Mention has already been made (p. 642) of a 

 sharp deep sulcus medius which runs longitudinally above the 

 posterior end of the optic chiasma. From this point a wide shal- 

 low groove runs forward under the lobus subhippocampalis to 

 the interventricular foramen. This groove resembles super- 

 ficially the sulcus Monroi of Reichert in the human brain and was 

 regarded by Herrick ('10, p. 470) as comparable with the sulcus 

 diencephalicus medius of amphibian brains. From the fact that 

 both the lobus subhippocampalis and the primordium of the cor- 

 pus striatum between which this sulcus lies are in Amphibia evag- 

 inated into the cerebral hemisphere, it follows of course that this 

 sulcus cannot as a whole be compared with either Reichert's sul- 

 cus Monroi or Herrick's sulcus medius. These sulci of higher 

 brains (which are probably incompletely homologous), however, 

 connect with the sulcus limitans of His in much the same way as 

 does the longitudinal sulcus here in question. For descriptive 

 purposes the term 'sulcus medius' will be applied to the entire 

 length of this sulcus of Ichthyomyzon, with the reservation, how- 

 ever, that its homology with the amphibian sulcus so named is 

 unproved and that in any event its anterior end cannot be so 

 regarded. 



