NERVOUS SYSTEM OF FISHES. 197 



parietal bone, is formed by the union of a branch from the facial 

 portion of the fifth with a branch of the vagus : tlie proper lateral 

 nerve is formed exclusively by the vagus, and divides into a 

 superficial branch supplying the lateral line, and a deep-seated 

 branch, communicating with the spinal nerves, and supplying 

 the myocommatal aponeuroses and the skin. * Whether the vagus 

 forms the whole or a part of the ' nervus lateralis ' it transmits 

 it from the fore part of its origin : the ' nervus accessorius ' when 

 present, which is rare in fishes, forms the hindmost part of the 

 vagus, as in higher Vertebrata. The nervus lateralis chiefly 

 supplies the myocommata, vertical fins and mucous line, pecu- 

 liarly ichthyic parts either by their preponderating development, 

 or their very existence : the nervus accessorius in mammalia, 

 sends no branch to the 'spinalis,' ' semispinalis,' or ' longissimus, 

 dorsi ' — the reduced homologues of the dorso-lateral myocommata of 

 fishes, but exclusively supplies the 'cleido-mastoideus' and 'cucullaris,' 

 associating them with the respiratory actions of the thorax. The 

 nervus lateralis may be in some respects analogous to the accessorius ; 

 it is not homologous with it. 



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

 cular branches to the gill covers. The usually double roots of the 

 nervus vagus pass out, in most fishes, by a single foramen in the ex- 

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

 ganglionic : it is the true pneurao-gastric, supjilying the gills and 

 stomach ; in the Tunny the branchial nerves are remarkable for their 

 size and their radical ganglions. The hinder second origin is usually 

 non-ganglionic, and is the source of the supra-temporal, glosso- 

 pharyngeal and lateral nerves. Some filaments rising behind the 

 vagus have been traced to the parts surrounding the brain within 

 the cranial cavity. The intestinal terminal filaments of the vagus 

 in Osseous Fishes communicate freely with the sympathetic. Each 

 vagal nerve of tlie Sturgeon equals the spinal chord in size and rises 

 by numerous roots. The vagal nerve has numerous roots, and an 

 extensive tract of origin in the Sharks, in which a posterior fasciculus 

 (^jig. 55. 8'), representing the ' nervus accessorius,' can be best de- 

 monstrated. 



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 proportion of clear (organic) filaments 

 to the opacpie (animal) filaments in the vagus of fishes is much greater 



* xxiii. torn. i. pp. 325 — 327. 

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