262 AN INTRODUCTION TO ZOOLOGY 



centre is closely bound up with the tuber acusticum, from which the 

 eighth nerve arises. Furthermore, other reasons lead us to regard 

 the ear as a specialised portion of the lateral line series of sense 

 organs, and so we sometimes speak of these two large brain ganglia 

 and all the nerves directly related to them as the acustico lateralis 

 system. Of this entire complex only the auditory centre and nerve 

 are present in the higher Craniata, i.e. many of the Amphibia, all 

 the Reptiles, the Birds and the Mammals. 



The manner in which the mixed nerves are distributed 

 also calls for notice. It will be seen that they have a very similar 

 arrangement. Each consists, when reduced to its simplest expres- 

 sion, of three principal parts. One going to the front of a gill cleft, 

 and so termed the pre-branchial or pre-trematic branch, well shown 

 in the seventh, ninth and tenth nerves. Another, the post-branchial, 

 or post-trematic branch, lies behind the same gill slit. The third 

 branch is situated more dorsaUy and passes forward, e.g. the 

 palatines of the seventh and ninth. It is clear, then, that the tri- 

 geminus is related to the mouth in the same way that the others are 

 to the gill clefts, so that for this and other reasons the mouth is 

 regarded by some authorities as representing the fusion of an anterior 

 pair of gill slits. In order to indicate this characteristic method of 

 distribution of the mixed nerves they are sometimes spoken of as 

 branchiomeric nerves. 



Spinal Nerves. 



The spinal nerves only call for brief notice. The typical 

 nerve arises from the cord by two roots, a dorsal and a ventral, of 

 which the former bears a ganglion and arises slightly behind the 

 latter. A pair of such nerves is present in each somite of the body. 

 The five anterior spinal nerves, together with three small so-called 

 spino-occipital nerves that come off by single roots from the hinder 

 end of the medulla in line with the ventral roots, join to form one 

 trunk. The nerve so constituted is distributed partly to the ventral 

 surface of the pectoral fin and partly to the sub-pharyngeal muscula- 

 ture. Spinal nerves 6-n approximate closely to one another to 

 form a very rudimentary sort of brachial plexus, and then pass on 

 to supply the pectoral fin. 



Sympathetic Nerves. 



These are quite inconspicuous in Scyllium, but consist of 

 two longitudinal chains bearing very small ganglia and lying on the 

 dorsal wall of the abdominal cavity, one on each side of the vertebral 

 column. 



