MEDULLA OBLONGATA 1G5 



which descends from the inferior colhculus — tr. tecto-bulbaris pos- 

 terior (figs. 1)2, 13, 29-34, 89, 90, tr.t.h.p.). Their segregation from 

 those of the general bulbar lemniscus in the urodeles seems to be de- 

 termined primarily by their terminal distribution; they end in the in- 

 ferior colliculus, while the larger bulbar lemniscus ends chiefly in the 

 superior colliculus. 



This tract is probably the precursor of the lateral lemniscus, which 

 first makes its appearance in definitive form in anurans. A "lateral 

 lemniscus" has been described in the brains of fishes by many au- 

 thors, but this is a misnomer. That large tract of fishes is comparable 

 with the general bulbar lemniscus of urodeles and has none of the dis- 

 tinguishing features of the mammalian lateral lemniscus, though it 

 doubtless includes the primordium of that tract. Its current name, 

 "f. longitudinalis lateralis," is more appropriate. 



Both these bulbar lemniscus tracts are well developed in Necturus 

 and the reader is referred to their description in that animal for dis- 

 cussion of the phylogenetic relationships ('30, pp. 51 ff.; Ariens 

 Kappers, Huber, Crosby, '36). Necturus lacks a cochlear rudiment, 

 and hence the lateral bulbar lemniscus here probably has a very im- 

 perfect auditory function, if any. In Ambly stoma and most other 

 urodeles there is such a rudiment, and here audition of a primitive 

 type may be represented, along with other functions, in this tract. In 

 adult Anura there is a well-developed primordium of the cochlea, 

 from which a cochlear division of the VIII nerve arises. Correlated 

 with this, there are cochlear nuclei, from which a true lateral lemnis- 

 cus passes to the inferior colliculus. Parallel with this differentiation, 

 both general and lateral lemniscus tracts, as seen in urodeles, are 

 radically reorganized. Larsell ('34, p. 521), after thorough examina- 

 tion of this question, accepts my interpretation of the lateral tract 

 of urodeles as an incipient lateral lemniscus, citing my remark ('30, 

 p. 58) : "It may be that this incipience of an auditory lemniscus in 

 Necturus is a central anticipation of a later peripheral specialization 

 of the cochlear rudiment— ^that is, an apparatus for sorting out 

 centrally the meager auditory component of the mixed functional 

 complex of the undifferentiated VIII nerve. But it is more likely an- 

 other evidence that Necturus is a degenerated or an arrested deriva- 

 tive of some more highly differentiated ancestor." A good brief sum- 

 mary of the evolution of the auditory apparatus has been given by 

 Papez ('36). For further comments on this tract see pages 188, 214. 



