182 THE BRAIN OF THE TIGER SALAMANDER 



These fibers appear to be related with cells bordering the velum, but 

 no clear evidence of their connections has been found ('36, p. 343; 

 '42, pp. 255, 291). Within and adjoining the anterior medullary 

 velum are fibers of passage of several kinds, including the decussa- 

 tion of the IV nerves, mesencephalic root of the V nerve, and tractus 

 tecto-cerebellaris. 



INTERMEDIATE ZONE 



The intermediate zone as defined here is represented by the small 

 gray area of the superior secondary visceral-gustatory nucleus and 

 associated neuropil. This gray lies dorsally of the isthmic tegmentum 

 and incompletely separable from it. In figures 2 and 13 it is shown 

 wedged between the isthmic tegmentum below and the nucleus cere- 

 belli and dorsal tegmentum above. The cells of the superior visceral 

 nucleus are of medium size, scattered and clumped in an open neuro- 

 pil, with a few outlying cells in the alba. Their dendrites are directed 

 ventrolaterally through the alba into the posterior isthmic neuropil 

 (described below), where they engage terminals of the ascending vis- 

 ceral tract ('42, pp. 244, 253, and fig. 43) and of fibers from many 

 other sources. 



There is an obscure vestige here of the nucleus isthmi, which is 

 well differentiated in the frog ('42, pp. 244, 253). In anurans and 

 reptiles this nucleus is in intimate relations with the lateral lemnis- 

 cus, and it is probably part of the auditory reflex apparatus, with, 

 perhaps, vestibular connections also. In Ambly stoma the auditory 

 system is poorly developed, and the crawling habit maintains the 

 balance of the body without nervous control; the nucleus isthmi, ac- 

 cordingly, is undeveloped. 



MOTOR ZONE 



This field is so intimately related anatomically and physiologically 

 with the tegmentum of the trigemino-facialis region that these will 

 be described together. The tegmentum isthmi is bounded anteriorly 

 by the isthmic sulcus and fovea isthmi, separating it from the cere- 

 bral peduncle and dorsal tegmentum. Posteriorly it is bounded by the 

 nucleus cerebelli and the trigeminal tegmentum. Its gray substance 

 includes the nucleus of the IV cranial nerve (as described in chap, x) ; 

 a compact central nucleus of the isthmus (fig. 29, nuc.is.c.) ; and, sur- 

 rounding this, a pars magnocellularis which is continuous spinal ward 

 with the large-celled component of the trigeminal tegmentum 

 (fig. 30). 



