642 C. JUDSON HERKICK AND JEANNETTE B. OBENCHAIN 



In the anterior part of the medulla oblongata there are two 

 longitudinal sulci, a shallow more medial groove separating the 

 somatic motor column from the lateral or visceral motor column 

 and a very deep lateral sulcus separating the lateral motor col- 

 umn containing the motor V nucleus from the somatic sensory 

 column containing the area acustica and the sensory V nucleus. 

 The lateral sulcus is the sulcus limitans of His, whose further 

 relations will next be considered. 



Sulcus limitans of His. The position of this sulcus can be 

 readily followed from the medulla oblongata to the level of the 

 tuberculum posterius at the rostral border of the mid-brain. Un- 

 der the cerebellum the strong eminence formed by the motor V 

 nucleus flattens out and the sulcus limitans becomes a wide shal- 

 low groove which bends upward above the fifth pair of giant 

 cells of Miiller (fig. 3). In the caudal end of the mid-brain the 

 sulcus disappears for a short distance, but its morphological posi- 

 tion can be clearly determined from internal evidence. In this 

 part of its course it descends to a point above the fourth pair of 

 Miiller cells and from here forward again is externally visible as 

 a shallow but sharply defined groove. Under the posterior com- 

 missure it rises abruptly to clear the third pair of Miiller cells, 

 in front of which it drops again. In front of the first pair of 

 MuUer cells it ascends gently to a point above the anterior end 

 of the tuberculum posterius. Here it disappears behind the lobus 

 ventralis thalami. 



There is a deep sharply defined depression above the posterior 

 end of the chiasma ridge which is termed sulcus medius thalami. 

 The posterior end of this depression lies in line with the stretch of 

 the sulcus limitans last described, though it is separated from it 

 by the lobus ventralis thalami, and internal evidenpe suggests 

 that it is a forward continuation of the sulcus limitans of His. 

 Under the caudal end of the sulcus medius there is a depressed 

 area (more obvious on the left side than on the right) extending 

 downward and forward into the recessus preopticus which prob- 

 ably represents the anterior end of the sulcus limitans. 



Reviewing the sulcus limitans as a whole, we find it very deep 

 in the anterior end of the medulla oblongata, shallow and wide 



