THE NERVOUS SYSTEM 153 



scribed above is to be regarded as the R. post-trematicus only, since 

 he finds in the larva an exceedingly small R. pre-trematicus, only 

 distinguishable as such by its position relative to the gill-slit, so that, 

 even supposing that the nerve persists after metamorphosis — which 

 is doubtful — it is quite unrecognizable after the gill-slit has dis- 

 appeared. 



(2) Fifth visceral arch nerve (n.vis.5). 



3rd Gill-arch nerve . , . . . . . Druner. 



This nerve is very similar to the one just described and, as already 

 mentioned, it is usually combined with it for a greater or lesser 

 distance from the ganglion outwards. It passes round the oesophagus 

 between the systemic and the 'third' arterial arches, and represents 

 only the post-trematic branch. It is almost certain that the pre- 

 trematic branch is wanting in the adult. Soon after leaving the gang- 

 lion it gives the following five branches. 



{ii) R. muscularis for the M. cephalo-dorso-subpharyngeus (c.ph.). 

 This branch leaves the nerve posteriorly and enters the muscle from 

 its mesial side. 



{h) RR. pharyngei, corresponding to those of the previous nerve, 

 come off very soon after, as do also : 



(c) RR. cutanei jugulares to the overlying skin. 



The remainder of the nerve may now divide, as did the previous 

 one, into: — 



{d) R. muscularis., passing ventral to the systemic arch to join the 

 corresponding branch of the fourth visceral nerve and supply the 

 M. subarcualis rectus I, 



(<?) R. recurrens (n.rec.5) passing dorsal to the aortae and distri- 

 buted to the floor of the pharynx. More usually, however, the former 

 branch is missing. 



(3) Truncus intestino-accessorius (Druner). The main branch of the 

 Xth nerve (n.tr.int.acc.) leaves the ganglion more posteriorly 

 than the two branches already described and passes in an oblique 

 postero-ventral direction, lateral to the M. opercularis, and mesial 

 to the MM. cephalo-dorso-subpharyngeus, dilatator laryngis, and 

 cucullaris. 



Very soon after leaving the ganglion the nerve gives off three 

 branches: 



(d) A R. muscularis anteriorly, which supplies the dorsal part of the 

 M. cephalo-dorso-subpharyngeus, and then posteriorly: 



