162 VISION 



(vr, Figure 23A) occupies the dorsolateral surface of the posterior lateral- 

 line lobe; both it and the posterior lateral-line nerve terminate among the 

 more deeply located cells, termed nucleus intermedins (ni, Figures 21-23). 



In Raja, a specialized plate of neurons is located within the nucleus 

 intermedius and is termed nucleus x (Figures 22, 23). Nucleus x may be only 

 an area of increased density within the intermedius, or it may be the source 

 of efferent fibers to the statoacoustic and lateral-line nerve receptors. A 

 similar but more ventrally located nucleus (nucleus B), has been described 

 in Squalus and Scyliorhinus (Smeets and Nieuwenhuys 1976). 



Without experimental data, it is at present impossible to define the 

 ventral border of nucleus intermedius in Raja because both the ventral root 

 of the anterior lateral-line nerve and the roots of the statoacoustic nerve 

 enter at the same level (Figure 22D). 



At this same level, an oval nucleus of large bipolar cells (mn, Figure 22D) 

 is located medially among the entering fibers of the statoacoustic nerve. This 

 nucleus is termed the magnocellular nucleus of VIII, and it is probably 

 homologous to the lateral vestibular nucleus (Deiter's nucleus) in other 

 vertebrates. A similar nucleus, termed the magnocellular vestibular nucleus, 

 is known in Squalus and Scyliorhinus (Smeets and Nieuwenhuys 1976). 



Upon entering the medulla, some fibers of the statoacoustic nerve 

 terminate in the magnocellular nucleus, while others form ascending and 

 descending tracts. The descending tract can be followed caudally into the 

 obex region (de, Figures 21, 22). Small- to medium-sized triangular cells 

 scattered among the descending statoacoustic fibers have been designated the 

 descending nucleus of VIII. No experimental data exist on the projections of 

 the statoacoustic nerve in Raja, but preliminary experiments on the stato- 

 acoustic nerve in fetal Squalus (McCormick and Northcutt, unpublished 

 observations) reveal a distinct descending pathway, which reaches a medullar 

 level comparable to that shown in Figure 22A. A comparable statoacoustic 

 pathway also exists in Amia (McCormick 1977) and can be traced to a 

 medullar level comparable to that shown in Figure 21A in Raja. 



Rostrally (Figure 23), as nucleus intermedius moves dorsally, a new 

 nucleus (dn, Figure 21A, B, C) begins to develop. This nucleus I have 

 tentatively identified as the dorsal nucleus of VIII in Raja, based on prelimi- 

 nary experiments in Squalus, and it may be homologous to part of the 

 cochlear nuclear complex in land vertebrates. Smeets and Nieuwenhuys 

 (1976) recognized a possibly similar cell group in Squalus and Scyliorhinus 

 and termed it the superior vestibular nucleus. 



Experimental study of the central connections of the lateralis and stato- 

 acoustic systems in cartilaginous fishes is only beginning. However, lateralis 

 input to the telencephalon has been demonstrated (Piatt et al. 1974). Given 

 the excellent sound detection and localization abilities of elasmobranchs 

 (Popper and Fay 1977), it is likely that these fishes will possess well- 

 developed ascending pathways related to these modalities. Perhaps one of 

 the most interesting questions is whether elasmobranchs possess separate 

 telencephalic projection areas for ampullary, ordinary lateral-line, and 

 acoustic information. Such a possibility would have seemed impossible 10 



