168 THE BRAIN OF THE TIGER SALAMANDER 



to that seen in Amblystoma. This makes it probable that in Amblys- 

 toma the fibers of the secondary tract that pass through the isthmus 

 and go directly forward to the peduncle and hypothalamus are gen- 

 eral visceral in function and that those that take the more dorsal 

 course to the superior nucleus are chiefly gustatory in function, 

 though this distinction may not be sharply drawn. In support of this 

 conclusion, Barnard ('36, p. 595) mentions the fact that this supe- 

 rior nucleus disappears in birds, in which the gustatory apparatus is 

 greatly reduced. If we may accept the suggestion of Fox ('41, p. 418) 

 and others that the ventral tegmental nucleus of mammals is com- 

 parable with the superior visceral-gustatory nucleus of lower forms, 

 it is evident that the mammalian nucleus has a more posteroventral 

 position, similar to that of the generalized ganoid fishes. 



The isthmic secondary visceral nucleus was first identified in 

 larval Amblystoma ('14a, pp. 364, 373) and subsequently in Nectu- 

 rus ('17, p. 248; '30, p. 62). Its cells form a low ventricular eminence 

 at the posterior dorsal tip of the isthmic tegmentum (fig. 2B). They 

 are of medium size, scattered and clumped in an open neuropil 

 (fig. 34), with outlying cells dispersed in the alba. As seen in trans- 

 verse sections, they lie immediately ventrally of the posterior end of 

 the isthmic sulcus ('25, fig. 19; '42. fig. 43). Their long dendrites are 

 directed laterally and ventrally into the posterior isthmic neuropil, 

 where they engage terminals of the secondary visceral tract, ascend- 

 ing root fibers of the trigeminus, tr. bulbo-tectalis lateralis, tr. bulbo- 

 isthmialis (fig. 38), collaterals of tr. tecto-bulbaris rectus (fig. 37; 

 '42, fig. 67), and terminals of the dorsal tegmental fascicles, notably 

 those of group (9), as described in chapter xx, from the cerebral hemi- 

 spheres. There is also a diffuse connection with the cerebellar forma- 

 tion, and this is much more intimate in some fishes ('05) . 



An extension of the secondary visceral-gustatory tract to the tec- 

 tum has been described by Brickner ('30) in some teleostean fishes. 

 In these fishes there is a large commissure connecting the two vis- 

 ceral-gustatory nuclei, which includes many decussating fibers of the 

 secondary visceral-gustatory tract. A separate fascicle of the latter 

 fibers turns forward, as tr. gustato-tectalis, to reach the anterior part 

 of the tectum opticum. This connection has not been observed in 

 Amphibia, but it may exist. It makes provision for correlation of 

 visceral-gustatory with visual experience, and it is accompanied by 



