MEDULLA OBLONGATA 167 



are those of the VII root. Most of them descend without division in 

 the f. soHtarius, but some divide with branches, which ascend for 

 short distances (fig. 37; '446, figs. 14, 17). 



In Amblystoma, as in all other vertebrates, the two f . sohtarii con- 

 verge at the calamus scriptorius into the commissural nucleus, and 

 part of their fibers decussate here in the dorsal commissura infima of 

 Haller. Here at the bulbo-spinal junction there is an important field 

 of correlation between the visceral-gustatory systems and the gen- 

 eral somatic sensory systems of the entire body. The details of this 

 structure in Amblystoma, the comparative anatomy of this region, 

 and its strategic importance for fundamental physiological problems 

 have recently been discussed ('446) and are summarized here in 

 chapter ix. 



The nucleus of the f. solitarius contains some neurons with den- 

 dritic connections exclusively with this fasciculus, but most of these 

 elements have other connections also. Some of their axons cross in 

 the ventral commissure and are lost to view in the vicinity of the 

 general bulbar lemniscus; others ascend uncrossed in the secondary 

 visceral-gustatory tract to the isthmus, midbrain, and hypothala- 

 mus. Some of these secondary fibers bifurcate, with branches taking 

 both these courses (fig. 9) . The first of these pathways is probably 

 more primitive, for Barnard ('36, p. 513) writes : "There is no differ- 

 entiated, uncrossed secondary gustatory tract in the lamprey. All 

 connections with higher centers are through the bulbar lemniscus 

 system." It is believed that in Amblystoma the decussating fibers 

 are concerned chiefly with bulbar reflexes; whether any of them as- 

 cend to higher levels in the bulbar lemniscus is not clear. In any 

 event the uncrossed tract is evidently the chief pathway to higher 

 centers. Rostrally of the V roots this tract divides, some of its fibers 

 continuing forward to area ventrolateralis pedunculi and some turn- 

 ing dorsad to the superior visceral nucleus in the isthmus. We have 

 many elective preparations of these fibers which have been cut in 

 various planes, and the entii-e course is clear. 



In the ganoid fishes (Johnston, '01; Barnard, '36, p. 517) the sec- 

 ondary visceral tract takes essentially the same course as in Amblys- 

 toma; but the superior visceral nucleus lies more ventrally and pos- 

 teriorly, under the cerebellum. In those teleosts in which the gusta- 

 tory system is enormously enlarged (carp and catfish), the secondary 

 visceral-gustatory nucleus shows corresponding enlargement and is 

 displaced forward and dorsalward in the isthmus to a position similar 



