SPINAL CORD AND BULBOSPINAL JUNCTION 129 



in the neuromuscular apparatus of these species. The primary fibers 

 evidently are concerned with massive movements of the trunk 

 musculature. The significance of the two sorts of fibers in the innerva- 

 tion of the limbs is still obscure. In Amblystoma the number of pri- 

 mary motor fibers is not markedly reduced by removal of the early 

 undifi^erentiated neural crest, while secondary fibers are, as a rule, 

 greatly reduced in number, the growth of the latter being dependent 

 on the presence of sheath cells and the former not (Yntema, '43a). 

 It will be of interest to learn whether the primary motor fibers have 

 functions in the embryogenesis of Amblystoma comparable with 

 those postulated for the "pioneer motor neurons" observed in the 

 bird by Hamburger and Keefe ('44, p. 237). 



THE BULBO-SPINAL JUNCTION 



Little need be added here to the general description of this impor- 

 tant region in chapter iv and to the details of structure and connec- 

 tions recently pubUshed ('446). The topography as seen in transverse 

 Weigert section is shown in figure 87. If the calamus scriptorius is 

 taken as the arbitrary boundary between spinal cord and brain, this 

 junctional region in Amblystoma may be considered to comprise the 

 segments of the first and second pairs of spinal nerves, the second 

 below the calamus and the first above. The entire length of the first 

 spinal segment overlaps the lower vagus region of the medulla 

 oblongata. 



In the sensory zone the somatic sensory systems of the neurons of 

 the dorsal gray columns are somewhat enlarged to form the nucleus 

 of the dorsal funiculi, which extends far forward in the lower vagus 

 region. Medially of this is the much larger collection of compactly 

 arranged smaller cells of visceral-gustatory function^ — the commis- 

 sural nucleus of Cajal. This nucleus extends downward from the 

 calamus for a distance of about one spinal segment, below which 

 visceral sensory function is represented by scattered cells in the 

 dorsal median raphe. Above the calamus the commissural nucleus 

 merges insensibly with the nucleus of the fasciculus solitarius. 



The funicular nucleus is regarded as comparable with the external 

 cuneate nucleus of mammals rather than with the nuclei of the f. 

 gracilis and f. cuneatus, since Amblystoma has no medial lemniscus 

 ('446, p. 318). The arrangement and connections of the commissural 

 nucleus are similar to those of man. 



Secondary fibers from the funicular nucleus (many of them myeli- 



