216 PISCES. 



neath the fin-rays to the tail. Varieties occur in these lateral nerves 

 in the several genera of Fish. 



The number of spinal nerves differs greatly in the several genera 

 of Fish. The two to four pairs of nerves that succeed the most ante- 

 rior pairs, usually give branches to the anterior extremities ; the suc- 

 ceeding pair of nerves passes, in the Pectorales, generally to the 

 posterior extremities or ventral fins ; in the Jugulares these same 

 nerves bend in the direction forward ; and in the Ventrales they usu- 

 ally arise further back from the seventh to the tenth pair. In the 

 Rays, which have such large pectoral fins, more than twenty pairs 

 coalesce together in one trunk for supplying the same ; about eight 

 or nine pairs pass to the also large ventral fins. 



A Sympathetic nerve occurs in all the Osseous and Cartilaginous 

 Fishes, but is developed however in very different degrees. It runs 

 upon either side of the vertebral column, where it forms more or less 

 distinct ganglia, and enters into connexions with the spinal nerves. 

 It forms plexuses with the vagus, which accompany the arteries, and 

 are in several Fishes particularly distinct upon the stomach, swim- 

 ming-bladder, and sexual organs. It unites at the base of the skull 

 with the trigeminus and vagus, but unquestionably obtains twigs from 

 the other cerebral nerves. 



In the very anomalous Cyclostomi the cerebral nerves, for exam- 

 ple, the ocular nerves, become simplified in their condition. In Pe- 

 tromyzon the abducens is absent, its elements being united with the 

 oculo-motor, but the trochlear nerve is present. The nervus tri- 

 geminus appears to contain motor fibres, since it ramifies upon the 

 muscles of the eye. A really distinct facial nerve also appears to 

 exist. 



The Myxinoidse have only six pairs of cerebral nerves, the olfac- 

 tory, optic, trigeminal, facial, acoustic, and vagus ; the ocular nerves 

 a*e absent with their corresponding muscles, the eyes being merely 

 rudimentary. 



Lastly, in Amphioxus all the cerebral nerves appear reduced to 

 the trigeminal. There arises, for example, from the cerebral end of 

 the spinal cord, a nerve somewhat thicker than the rest, that passes 

 to the snout of the animal ; it corresponds only to a part of the tri- 

 geminus, the greater portion of the head and sides of the mouth be- 

 ing supplied from the five succeeding spinal nerves. No branches 

 of spinal nerves appear to pass to the gills. 



The Cyclostomi exhibit the peculiarity of being destitute of a 



