METAMERISM. 



257 



In the facial segment the post-trematic ramus has the 

 typical communis and viscero-motor fibres and in addition 

 a large lateralis component and a small general cutaneous 

 bundle which joins it extra-cranially from the Gasserian 

 ganglion and distributes to the skin of the lower part of 

 the operculum. The two last are to be regarded as second- 

 ary additions, the former following the differentiation of 

 the operculo-mandibular canal and the latter the back- 

 ward growth of the operculum. That these general cuta- 

 neous fibres of bony fishes have not persisted from a 

 primordial condition in which cutaneous nerves were 

 normally present in branchial nerves is suggested by the 

 fact that they do not emerge with the facial root, but 

 swing back from the trigeminus. If the facialis ever 

 possessed a general cutaneous component properly its 

 own, in all known vertebrate types its ganglion has sec- 

 ondarily fused with the trigeminal general cutaneous 

 (Gasserian) ganglion. 



The pre-trematic facial is strictly typical, containing 

 only communis fibres. In some fishes (but not in 

 Menidia) this ramus seems to have secondarily fused after 

 the obliteration of the spiracle, with the r. mandibularis V 

 and to be represented in part by the chorda tympani of 

 higher vertebrates. 



The r. palatinus of the facial segment is also typical, 

 save that it is longer than in the other nerves. 



There is no general cutaneous lateral branch of the 

 facial segment, though possibly the r. buccalis may repre- 

 sent such a nerve to which lateralis fibres were added and 

 then the original general cutaneous component disap- 

 peared. For this no satisfactory evidence can be adduced. 

 The r. oticus, however, may represent a dorsal branch. 

 For we have seen that this nerve contains general cuta- 



