156 THE BRAIN OF THE TIGER SALAMANDER 



below — there is a similar lack of segregation of separate pathways for 

 the several modalities of sense. The general bulbar lemniscus {Im. of 

 the figures) carries fibers which may be activated from any or all of 

 the sensory nerve roots. The segregation of some specific sensory 

 pathways is incipient in the lemniscus systems, and this is seen with 

 especial clarity in the secondary ascending visceral-gustatory path 

 (tr.v.a.). 



INTERMEDIATE ZONE 



The reticular formation of the spinal cord is continued forward into 

 the medulla oblongata (fig. 9; '14a, p. 378; '30, p. 59). In Weigert 

 sections (figs. 89, 90) it may be recognized as a lighter field between 

 the bulbar lemniscus (Im.) and the spinal lemniscus (Im.sp.). Here the 

 spino-bulbar, spino-cerebellar, and spinal lemniscus tracts are as- 

 sembled in the alba. 



The neuropil of this field receives dendrites from the underlying 

 gray and from the adjoining sensory and motor zones (fig. 9; '14a, 

 figs. 22-43; '446, figs. 7, 8). Some of these cells are small, with local 

 connections; others are very large, with wide dendritic spread and 

 long axons (most of them myelinated), which descend as internal 

 arcuate fibers to the ventral funiculi. Many of them decussate in the 

 ventral commissure; and both crossed and uncrossed fibers, before 

 or after crossing, bifurcate into ascending and descending branches. 

 The descending bulbo-spinal fibers and the ascending bulbo-tegmen- 

 tal fibers are part of the neuromotor apparatus of bulbar and spinal 

 reflexes. The lemniscus systems, on the other hand, arise chiefly in the 

 sensory zone, though some of their fibers may be axons of neurons of 

 the intermediate zone, a distinction of no great significance, in any 

 event, because many neurons have extensive dendritic spread in both 

 zones. The lemniscus systems also differ from the neuromotor ap- 

 paratus just described, in their terminal connections, passing not to 

 the motor zone but to higher levels of the sensory zone. 



Among the large cells of the intermediate zone the two giant cells 

 of Mauthner are of special interest. They lie at the level of the 

 vestibular roots and have large dendrites directed outward among 

 terminals of these root fibers ('14a, fig. 12). In the 12-mm. larva they 

 are of enormous size ('14a, fig. 53), with thick dendrites, which ramify 

 throughout the entire area of the white substance. In the adult the 

 dendritic spread is less extensive, yet it is sufficiently wide to em- 

 brace the greater part of the alba of the intermediate and motor 

 zones and especially the terminal area of vestibular fibers. The thick 



