164 THE BRAIN OF THE TIGER SALAMANDER 



that the latter may be activated by a wider variety of sensory excita- 

 tions, including the vestibular, lateral-Hne, and visceral-gustatory 

 systems. This lemniscus is one of the largest tracts of the urodele 

 brain and is very large in fishes also. In anurans and higher forms it 

 can be recognized with difficulty because its fibers are dispersed or 

 segregated in other specialized tracts like the trigeminal lemniscus. 

 Amblystoma has nothing comparable with the medial lemniscus of 

 mammals. 



There is some evidence of an incipient trigeminal lemniscus, 

 though most of the secondary general cutaneous fibers ascend in the 

 general bulbar lemniscus ('39&, p. 606). From the region of the supe- 

 rior sensory V nucleus under the auricle a large number of arcuate 

 fibers descend to the ventral commissure. Some of these enter the gen- 

 eral bulbar lemniscus. Others are added from the entire length of the 

 spinal V nucleus, and in the calamus region similar fibers join the 

 spinal lemniscus. These connections were observed by Woodburne 

 ('36, p. 455). From the region of the superior V nucleus, fibers ascend 

 uncrossed as far as the isthmic tegmentum, where they end. This 

 may be the precursor of an uncrossed trigeminal lemniscus. In the 

 preparation from which figure 39 was drawn, no fibers of the lemnis- 

 cus systems are impregnated. The visible external and internal ar- 

 cuate fibers make local bulbar connections, descend to the spinal 

 cord, and ascend only as far as the isthmus. Some of these fibers di- 

 vide into ascending and descending branches before or after decus- 

 sation. Anteriorly on the right, a compact fascicle of external arcu- 

 ates descends from the superior trigeminal neuropil, turns forward, 

 and decussates close to the ventral surface within the interpeduncu- 

 lar neuropil. Most of them descend in the bulbo-spinal tract, but 

 some turn forward as tr. bulbo-isthmialis. This may be a precursor of 

 a separate crossed trigeminal lemniscus. Their course is parallel with 

 tr. bulbo-tectalis lateralis (primordial lateral lemniscus), but deeper. 



3. TRACTUS BULBO-TECTALIS LATERALIS (fIGS. U, 91, 9'2, tr.b.t.l.) 



The tr. bulbo-tectalis lateralis is a mixed system of fibers, closely 

 associated with the general bulbar lemniscus and of similar origin 

 from the sensory zone, chiefly from its middle part in the vicinity of 

 the vestibular nerve roots. In the isthmus it lies externally of the 

 general bulbar lemniscus, and it terminates exclusively in the primor- 

 dial inferior colliculus (nucleus posterior tecti) and the underlying 

 isthmic neuropil. These fibers are mingled with those of a system 



