1C6 THE BRAIN OF THE TIGER SALAMANDER 



4. THE SECONDARY VISCERAL-GUSTATORY TRACT {tr.V.a.) 



The fibers of the secondary visceral-gustatory tract are segregated 

 in terms of physiological specificity more definitely than are those of 

 the other lemniscus systems. They are axons of neurons of the nu- 

 cleus of the f . solitarius and commissural nucleus, which ascend un- 

 crossed ventrally of the spinal V root. Most of the visceral sensory 

 fibers of the VII root are gustatory, and most of those of the vagus 

 roots are general visceral. It is probable that the secondary fibers 

 arising more anteriorly are activated mainly from taste buds, but 

 these cannot be distinguished from those of general visceral sensibili- 

 ty. The central courses of these fibers have been fully described and 

 illustrated in the larva ('14a), in the adult ('446), and in Necturus 

 ('30). A useful summary of the comparative anatomy of this system 

 has been published by Barnard ('36). The adult arrangement is 

 shown here in figures 7, 8, 9, 23, 30, 37, 38, 87-90. 



One is impressed by the stability of the general plan of these ar- 

 rangements throughout the vertebrate series, despite the most ex- 

 treme modifications of the details in adaptation to different modes 

 of life. This applies particularly to the peripheral and bulbar connec- 

 tions. The ascending connections are less well known, and they 

 probably show more radical changes in the series from lower to 

 higher vertebrates. Fishes and amphibians exhibit a common plan, 

 with infinite variety of detail; the arrangement in Amblystoma ap- 

 pears to present this plan reduced to its simplest form, and this 

 generalized plan may be taken as a point of departure from which the 

 cerebral apparatus of visceral-gustatory adjustments of higher ani- 

 mals has been derived. 



All the peripheral fibers enter the f. solitarius, which spans the 

 entire length of the medulla oblongata. They are thin, and many of 

 them are myelinated. Most of those from the geniculate ganglion, 

 which form the visceral sensory VII root, have a T-form division, 

 with ascending and descending branches as they eiiter the f . solitari- 

 us. Most (perhaps all) of the prefacial f. solitarius is composed of 

 these ascending facialis fibers, which are less heavily myelinated than 

 are their descending branches and, presumably, are gustatory in 

 function. These fibers ascend to the auricle, where they end in the 

 same neuropil as the ascending V root (figs. 7, 30, 38; '14a, p. 365). 

 By this arrangement, correlation may be effected between visceral- 

 gustatory and general somatic sensibility of the mouth cavity. The 

 visceral sensory fibers of the IX and X roots are less myelinated than 



