303 



ANATOMY OF VEKTEBRATES. 



between the ex-occipital and tlie 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, i, the retractor hyoidei, c, and 

 geniohyoidei, 27, fig. 135: It is called 'hypoglossal nerve' by 

 some lehthyotomists ; but this name more properly applies to a 

 nerve which, in some fishes, arises from the medulla oblongata 

 behind the vagus, is distributed to the muscles between the scapular 

 and hyoid arches, and unites with the first spinal nerve. 



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

 or motory origin ; sometimes each rises singly ; sometimes, as in 

 the Cod, by two or more filaments, fig. 196. Both sensory and 

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

 largest, arises 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 

 branch of the S23inal 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 commonly upon the whole sensory origin of the nerve, 

 where it emerges from the neural canal. In some fishes (Bream 

 and Garj)ike) 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 emer- 

 gence, and the junction of the two roots takes 

 place quite exterior to the neural canal. 



The connection of the roots with the myelon 

 is weaker in Fishes than in air-brcathino- animals: 

 it is so easily broken in the Dcrmopteri as to have 

 led to a denial of its existence.' The pecidiar 

 combination of the dorsal and ventral roots of tlie 

 spinal nerves in Osseous Fishes is well seen in the 

 Cod.^ Tlie dorsal root sends a filament, fig. 204, a, 

 upward, which joins a ventral filament, h, from 

 the preceding nerve, and forms the ramus dor- 

 salis, d; the dorsal root sends two filaments, c, downward, which 

 unite together, and with a ventral filament, c, of the same nerve 



204 



C'unTioitiuni nf pplii il 

 and Iitr 1 a iKi\(.^, 



( 11,1 Ll\ 



Lxxix. ii. p. 47'J. 



LIV. pi. X. 



Jj 



