184 THE BRAIN OF THE TIGER SALAMANDER 



rect their dendrites laterally among terminals of the ascending sen- 

 sory V root and related tracts ('396, figs. 47-49, 53, m, 68). The 

 axons of some of these elements are directed forward, as shown in 

 figure 43. These short fibers pass from the trigeminal tegmentum to 

 arborize within the gray of the isthmic tegmentum and are doubtless 

 part of the apparatus of intrinsic co-ordination of patterns of action 

 within the motor zone. 



Some of the small cells at the anterior end of the trigeminal teg- 

 mentum are shown in figures 66 and 91. These resemble those of the 

 adjacent interpeduncular nucleus. Their dendrites extend laterally 

 through the entire thickness of the alba of the ventral part of the 

 auricle, where they are in contact with all kinds of fibers passing this 

 region. The axons of some of them descend to the interpeduncular 

 neuropil (figs. 63, 80, 84; compare similar cells of the isthmic teg- 

 mentum, figs. 60, 84). 



ISTHMIC TEGMENTUM 



Horizontal sections are most instructive in the analysis of this re- 

 gion (figs. 29-33). The nervous elements are arranged in five groups: 

 (1) A narrow and inconstant layer of subependymal cells. (2) These 

 are separated by a thick sheet of neuropil from the lens-shaped area 

 of small and medium cells, here termed the "central nucleus of the 

 isthmus." (3) Externally and spinalward of this area is the pars 

 magnocellularis, containing scattered nerve cells of medium or large 

 size. The extensive neuropil within which these cells are imbedded is 

 broadly continuous with that of the surrounding alba. This large- 

 celled area envelops the lentiform area of small cells laterally, ven- 

 trally, and caudally and is continuous spinalward with a similar field 

 of the trigeminal tegmentum. In figure 2B the dotted line separating 

 the pars magnocellularis (teg.is.m.) is arbitrarily drawn to indicate a 

 posterior sector which may be assigned to either isthmic or trigemi- 

 nal tegmentum. (4) Dorsally of all the preceding, the gray of the 

 isthmic sector extends upward into contact with that of the dorsal 

 tegmentum and ventral cerebellar nucleus. This area is the superior 

 secondary visceral-gustatory nucleus (fig. 2B, iiuc.vis.s.). (5) A prob- 

 able vestige of the nucleus isthmi. 



The first component of the preceding list is the ventral part of the 

 zone of deep subependymal gray, which everywhere surrounds the 

 aqueduct. In the isthmus it is thin and contains few nerve cells, but 

 more posteriorly in the trigeminal tegmentum it expands to a wide 

 zone of subependymal small cells (figs. 29, 30). This and the associ- 



