NERVES. 107 



men, situated in Teleosteous fishes in the exoccipital ; and 

 form ganglionic swellings, of which those of the lower stem 

 are the more conspicuous. The lower stem has mixed elements, 

 motory as well as sensory, and is distributed to the muscles 

 of the branchial arches and pharynx, the oesophagus and 

 stomach; it sends filaments to the heart and to the air- 

 bladder where it exists. The first (upper) stem forms the 

 Nervus lateralis. This nerve, which accompanies the lateral 

 mucous system of the trunk and tail, is either a single longi- 

 tudinal stem, gradually becomiag thinner behind, running 

 superficially below the skin (Salmonidse, Cyclopterus), or deeply 

 between the muscles (Sharks, Chimsera), or divided into two 

 parallel branches (most Teleostei) : thus in the Perch there 

 are two branches on each side, the superficial of which sup- 

 plies the lateral line, whilst the deep-seated branch communi- 

 cates with the spinal nerves and supplies the septa between 

 the myocommas and the skin. In fishes which lack the lateral 

 muciferous system and possess hard integuments, as the Ostra- 

 cions, the lateral nerve is more or less rudimentary. It is 

 entirely absent in Myxinoids, but the gastric branches of the 

 Vagus are continued, united as a single nerve, along the intes- 

 tine to the anus. 



No fish possesses a Nerous accessorius. Also a separate 

 Nervus hypoglossv^ {twelfth pair)^ is absent, but elements 

 from the first spinail nerve are distributed in the area 

 normally supplied by this nerve in higher vertebrates. 



The number of Spinal nerves corresponds to that of the 

 vertebrae, through or between which they pass out. Each 

 nerve has two roots, an anterior and posterior, the former of 

 which has no ganglion, and exclusively contains motor 

 elements. The posterior or dorsal has a ganglionic enlarge- 



1 Miiller considers a nerve rising jointly with the Vagus in Petromyzon to 

 be this nerve (Fig. 45, hy). 



