74 THE LOWER PRIMATES 



its fellow of the opposite side by the dorsal septum. The base remains 

 narrow while the distal portion presents a tendency to swing and spread 

 laterally. 



At a sHghtly higher level the first indication of the sensory nucleus of 

 Burdach appears as a flat, bilateral thickening in the dorsolateral portion 

 of tlie central gray cohmin. The two nuclei then extend dorsally with the 

 same lateral swing which is characteristic of the dorsal gray masses and of 

 the substantia gelatinosa Rolandi preparatory to the opening of the fourth 

 ventricle. As these nuclei of GoII and Burdach increase in size, they develop 

 at their dorsal extremities overhanging masses of nuclear material, almost 

 arboreal in form, but limited to the lateral aspects of the main trunks of the 

 nucleus. The more lateral of these nuclear appendages forms the external 

 nucleus of Burdach. Its Icaf-hke appearance is produced by the breaking-up 

 of the nuclear material by the bundles of fibers of the column of Burdach 

 which are here seeking their cells of relay. The substantia gelatinosa of 

 Rolando which forms the cap surmounting the dorsal horns passes insensibly 

 into the substantia gelatinosa trigemini with a marked lateral inclination 

 in accordance with the common lateral swing of these nuclear columns, until 

 it almost reaches the lateral meridian. As this swing takes place, the substan- 

 tia gelatinosa Rolandi, slender in the spinal cord, rapidly increases in size to 

 become the substantia gelatinosa trigemini of the oblongata. These three 

 dorsal columns of gray matter expand and diverge in almost parallel para- 

 bolic rows. The substantia gelatinosa trigemini approaches the lateral sur- 

 face of the cord and, losing its intimate contact with the reticular formation, 

 extends upward in a position so constant that it may be used as an orient- 

 ing structure in the study of the stem. The nuclear gray column reaches 

 upward to the midpontine level where dorsomesial to it there appears the 

 motor nucleus of the trigeminus nerve. At the midolivary level the sub- 

 stantia gelatinosa trigemini presents a definite constriction which has been 



