188 THE BRAIN OF THE TIGER SALAMANDER 



lemniscus systems; vast numbers of terminals of descending fibers 

 of the tegmental fascicles; and collaterals of tr. tecto-bulbaris rectus. 

 This evidently is a strategic region in the organization of both as- 

 cending and descending systems of conduction. Within this undiffer- 

 entiated field the primordia of a considerable number of specific nu- 

 clei of other animals can be recognized, including specialized struc- 

 tures peculiar to fishes and a number of mammalian nuclei. Among 

 these components (with their probable derivatives in more special- 

 ized species) the following can be identified: 



1. The chief sensory nucleus of the trigeminus. Some ascending 

 sensory V root fibers terminate here, though most of them more pos- 

 teriorly, and some pass through to reach the commissura cerebelli 

 and the corpus cerebelli of the same and of the opposite side. Secondary 

 fibers go from this area upward into the cerebellum and forward with 

 or without decussation as the primordial trigeminal lemniscus. 



2. Similarly, some of the ascending root fibers of the VIII and 

 lateral-line nerves terminate more laterally in the posterior isthmic 

 neuropil (superior vestibular nucleus), and some of the VIII fibers 

 reach the lateral (auricular) part of the cerebellum of the same and of 

 the opposite side, the latter decussating in the com. vestibulo-later- 

 alis cerebelli. 



3. The longitudinal bulbar correlation tracts a and b reach the 

 ventral part of the same neuropil. This field is incorporated within 

 the cerebellum of mammals. 



4. Spinal lemniscus fibers, as they recurve dorsal ward along the 

 anterior face of the auricle to reach the tectum, are spread through 

 the middle part of this neuropil, with collaterals and branched ter- 

 minals within it (figs. 42, 43, 44). 



5. Associated with the spinal and bulbar lemniscus and laterally 

 of them in the isthmus are fibers of the lateral bulbo-tectal tract 

 (fig. 11, tr.b.t.L), which is the probable primordium of the lateral 

 lemniscus. These ascending fibers are mingled with descending fibers 

 of tr. tecto-bulbaris posterior (fig. 12, tr.t.b.p.; '396, p. 606 and fig. 

 96). The position of the mixed fascicle in horizontal sections is shown 

 in figures 31-34, here marked tr.t.b.p. Terminals and collaterals of the 

 ascending lateral bulbo-tectal fibers spread throughout the posterior 

 isthmic neuropil (figs. 33, 34), and these endings are comparable with 

 those of the lateral lemniscus in the nucleus isthmi of the frog (Lar- 

 sell, '24). This field also contains precursors of some of the nuclei of 

 the lateral lemniscus of mammals, specifically the dorsal nucleus of 

 the lateral lemniscus (Ariens Kappers, '29, p. 304; Clark, '33). 



