3o8 THE INTERMEDIATE PRIMATES 



ing tract of the fifth or trigeminal nerve ( Trd ). Other structures of inter- 

 est at this level should be noted; among them, the ventral gray cohimn 

 ( Ven) cut otf from its former attachment to the central gray matter ( Cen) 

 by the crossing fibers of the pyramidal system (Pyx). 



Dorsal to the ventral gray column are the scattered bundles of the 

 pyramidal tract (Py) as they are beginning to make their descent from 

 the oblongata into the spinal cord. Passing through these scattered pyram- 

 idal bundles in a transverse direction are a few of the emergent fibers 

 of the eleventh pair of cranial nerves, the spinal accessory nerve. Along 

 the margin of the section is a narrow band of medullary substance, con- 

 stituting the circumferential zone. This consists of the major ascending 

 spinocerebellar fibers ( Fie, Gow ). Lying internal to the circumferential zone, 

 bordering upon the ventral gray column, is the intermediate zone containing 

 in the central portion among other tracts, the two descending Deiterso-spinal 

 tracts ( DT) and in its more lateral portion, the spinothalamic and rubro- 

 spinal tracts ( Spt, Rst). 



LEVEL OF THE CAUDAL EXTREMITY OF THE NUCLEUS OF BURDACH (FIG. 1 45) 



At this level an illustration is introduced to show more clearly certain 

 nuclear details in the dorsal sensory field. In this section, perhaps better than 

 elsewhere, morphological specializations in the dorsal columns are evident. 

 Thus in close relation to the dorsomedian septum is the nuclear mass 

 constituting the nucleus of Goll (NO), lateral to which is the nucleus of 

 Burdach (^NB), and finally in the most lateral position, the substantia 

 gelatinosa (NR). These three structures represent the relay stations for 

 all the sensory afferent fibers coming from the head and neck, upper 

 trunk and arm, lower trunk, leg and tail. Their preemption of the dorsal 

 field gives the region its predominantly sensory character. On either side of 



