198 LECTURE VIII. 



than in that nerve in higher Vertebrata, according to Bidder and 

 Volkmann : which illustrates the progressive character of the indivi- 

 dualisation (selbsstandigkeit) of the great sympathetic. 



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 peculiar erectile palatal organ of the Cypri- 

 noids is wholly, and the peculiar electric organs of the Torpedo are, in 

 great part, supplied by the very remarkable and characteristic vagal 

 nerve of fishes. 



The first spinal 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 between the ex-occipital 

 and the atlas, and divides into a small dorsal and a larger ventral 

 branch : this communicates with the ventral branch of the next 

 spinal nerve, and supplies the pectoral fin-muscles, the subcoradoideus, 

 and the sterno-hyoideus (44. /jy); it is called ' hypoglossal nerve ' 

 by some Ichthyotomists. 



Each of the true spinal nerves has a dorsal or sensory, and a 

 ventral or motory origin : sometimes each rises by a single root ; 

 sometimes, as in the Cod, by two or more roots. Both sensory 

 and motory roots are long in most fishes : the sensory root is the 

 largest, ai'ises by more filaments, and further back than the motory 

 roots in the Sturgeon. 



In most Osseous Fishes one dorsal root goes to form the dorsal 

 bi'anch of the spinal nerve, and the other dorsal root joins the 

 ventral branch of the same nerve : sometimes the ganglion is formed 

 on the dorsal root of the dorsal branch, as in the Cod ; more com- 

 monly upon the whole sensory origin of the nerve, where it emerges 

 from the neural canal. In some fishes (Bream and Gar-pike) the 

 ganglions on the dorsal root are situated in the spinal canal : more 

 commonly (as in the Cod, the Ling, the Sander) the ganglions are 

 external to the spinal canal. In both cases the nerve is increased in 

 size beyond the ganglion and the union of the ventral root. This is 

 well seen in the Skate, in which the ganglions are situated beyond 

 the holes of emergence, and the junction of the two roots takes place 

 quite exterior to the neural canal.* 



The connection of the nerve-roots with the myelon is weaker in 

 fishes than in air-breathing animals : it is so easily broken in the 

 Dermopteri, as to have led to a denial of its existence, f The best 

 illustration of the peculiar combination of the dorsal and ventral 



* Lxxvii. ii. p. 479. -j- liv. pl.x. 



