i 



NERVES OF FISHES. 307 



seated branch, communicating with the spinal nerves, and sup- 

 plying the myocommatal aponeuroses and the skin. 1 Whether 

 the vagus forms the whole or a part of the ' nervus lateralis,' this 

 does not arise like the ( nervus accessorius ' of higher Vertebrates, 

 from a motory tract of the myelon, but from a ganglionic part of 

 the vagus. The ' nervus lateralis ' chiefly supplies the skin, 

 mucous line, intermuscular septa, and vertical fins, most of them 

 peculiarly ichthyic parts, either by their preponderating develope- 

 ment, or their very existence. 



The vagus sends supra-temporal branches to the head, and 

 opercular branches to the gill-covers. The usually double roots of 

 the nervus vagus pass out, in most Fishes, by a single foramen in 

 the exoccipital bone. The fore part of the root is the largest, and 

 is ganglionic : it is the true pneumo-gastric, supplying the gills, 

 pharynx, heart, and stomach, and sending filaments to the sep- 

 tum dividing the branchial from the abdominal cavity. In the 

 Tunny the branchial nerves are remarkable for their size and 

 their radical g-ano-lions. The hinder second origin is the source of 

 the glosso-pharyngeal and lateral nerves. The former, which 

 has a distinct ganglion in the Herring and some other fishes, 

 supplies the first gill and contiguous parts, and thence passes 

 forward to the tongue. Some filaments rising behind the vagus 

 have been traced to the parts surrounding the brain within the 

 cranial cavity. 2 Each vagal nerve of the Sturgeon equals the 

 spinal chord in size, and rises by numerous roots. The nerve 

 has a like extensive tract of origin in the Sharks ; in which a 

 posterior fasciculus, fig. 187, 8, representing the 'nervus acces- 

 sorius,' can be best demonstrated. 



There is no ' nervus lateralis ' in the Myxinoids, but the gastric 

 branches of the vagus are continued, united as a single nerve, 

 along the intestine to the anus. The vagus is represented in the 

 Branchiostoma by a branch sent from the fifth to the pharynx. 

 In the Myxine its origin is close to that of the fifth. The 

 erectile palatal organ of the Cyprinoids is wholly, and the electric 

 organs of the Torpedo are, in great part, supplied by this remark- 

 able vagal nerve. The proportion of grey to white filaments 

 in the vagus of Fishes is greater than in that nerve in higher 

 Vertebrates, which illustrates the progressive differentiation of 

 the great sympathetic. 3 



The^rs^ spinal, or myelonal, nerve rises usually by two roots, 

 the dorsal one having a ganglion, rarely by non-ganglionic roots 

 exclusively from the prepyramidal tracts : it usually emerges 



1 xxiii. torn. i. pp. 325-27. 2 ccxxvii. 3 ccxn. 



x 2 



