996 



PISCES. 



through a foramen formed by the parietal and 

 interparietal bones, and runs along the whole 

 length of the back on each side of the dorsal 

 fins, receiving in its course filaments from all 

 the spinal nerves, and giving off branches 

 to the muscles and rays of the fins of the 

 back. This branch is superficial up to the 

 point where it plunges beneath the little ex- 

 ternal muscles of the fin-rays, and it sometimes 

 gives off branches which are equally superficial, 

 and that descend to the muscles of the trunk 

 above the pectoral fins, and others which run 

 backwards as far as the anal fin, where they 

 form a longitudinal nerve resembling that of 

 the back. Such is the general arrangement of 

 this remarkable nerve, but it is by no means 

 invariably so : thus, in the Carp it seems to 

 proceed from the eighth pair, and not from the 

 fifth. In the Silurus, on the contrary, it ema- 

 nates from the fifth alone, while in the Perch, 

 Cod, &c. it is derived, as has been described, 

 equally from both these sources. 



6. The sixth pair of nerves, or abdiic ens, (fg. 

 529, 6) takes its origin, as in other Vertebrata, 

 from the inferior surface of the medulla oblon- 

 gata, and is entirely appropriated to the external 

 rectus muscle of the eye. 



7. The seventh pair of nerves (fig. 529, 7) is 

 appropriated, as in other Vertebrata, to the 

 sense of hearing. It arises from the medulla 

 oblongata between the fifth and eighth pairs, 

 and is distributed over the sacculi which con- 

 tain the otolithes and the ampullae connected 

 with the semicircular canals of the ear. It has 

 likewise connections with the last branch of 

 the fifth pair, and one which is especially con- 

 stant with the glosso-pharyngeal division of the 

 eighth pair of nerves. 



8. The roots forming the eighth pair (fg. 529, 

 8), or nervus vagus, are collectively almost as 

 large as the fifth, behind which ihey take their 

 origin generally by numerous filaments that 

 issue in a single line, that runs longitudinally 

 along the sides of the medulla oblongata be- 

 neath the lobes situated behind the cerebellum, 

 and which unite into a ganglion (fg. 530, t) 

 before its divisions are given off. 



The distribution of the eighth pair of nerves 

 in Fishes affords a striking example of the con- 

 stancy with which a nerve presides over the 

 same functions in every class of vertebrate 

 animals. 



Ihe glosso-phari/ngeal issues from the cra- 

 nium sometimes through an aperture in the 

 lateral occipital, sometimes through a foramen 

 in the petrous bone, and supplies the first 

 branchia and the parts in its immediate vici- 

 nity, whence it passes forward to the tongue, 

 in which it is ultimately expended. 



The nervus vagus properly so called leaves 

 the cranium through a special foramen in the 

 lateral occipital bone, and soon dilates into a 

 large ganglion, from which nerves proceed to 

 supply the three last branchiae and the inferior 

 parts of the pharynx. The trunk of the nerve 

 then passes on along the pharynx and ceso- 

 phagus as far as the stomach, which it likewise 

 supplies. This distribution, as will be seen, 

 is similar to what is found to exist in all the 



vertebrate classes as far as relates to the func- 

 tions over which the nerve presides, although 

 its arrangement is necessarily modified in con- 

 sequence of the changed position of the respi- 

 ratory organs. The eighth pair of nerves gives 

 off one important branch, and sometimes two, 

 the relations of which with what is met with 

 in the superior classes are not so apparent. 

 The first of these is a branch which arises 

 sometimes from the anterior roots of the vagus, 

 and sometimes from the posterior margin of its 

 ganglion, and runs in a straight line as far as 

 the tail. In many Fishes, after having given 

 off a superficial filament which follows the 

 commencement of the lateral line, the trunk 

 of the nerve passes straight backwards im- 

 bedded in the thickness of the lateral mus- 

 cles, between the ribs and their appendices, 

 receiving special filaments from every one of 

 the spinal nerves quite distinct from the inter- 

 costals, and giving off branches to the skin, 

 which pass through all the intervals between 

 the muscular layers. In other cases, as in the 

 Cod-fish represented in the figure, it is super- 

 ficial throughout its whole course, and appa- 

 rently has no communication with the spinal 

 nerves, although perhaps such communications 

 may exist in the shape of very delicate fibrillaa. 



The second remarkable branch is that already 

 described as joining an offset from the fifth to 

 form the dorsal nerve. 



The eighth pair likewise gives off filaments 

 to the diaphragm or membranous septum 

 which divides the branchial chamber from the 

 abdominal cavity. 



The last pair of cranial nerves arises behind 

 the eighth pair from the medulla oblongata, 

 and, after giving a branch to the swimming 

 bladder, is distributed to the muscles of the 

 shoulder, and those which pass between the 

 shoulder and the hyoid apparatus ; it also gives 

 branches which anastomose with those of the 

 first spinal nerve, and from the plexus thus 

 formed the nerves proceed which supply the 

 external and anterior muscles of the pectoral 

 fins. 



The second pair of spinal nerves supply the 

 internal and posterior muscles of the pectoral 

 fins. In the Triglae (G urn arch) these nerves 

 are remarkable for their great size, and on ac- 

 count of the large branches that they give off to 

 the free rays situated in front of the pectorals. 

 They arise from the sides of the last of the five 

 pairs of post-cerebellic lobes, which in this race 

 of Fishes are so remarkable. 



In Fishes which have their pelvis suspended 

 to the bones of the shoulder, whether the ven- 

 tral fins appear in front of the pectorals or be- 

 neath them, or behind them, it is from the third 

 and fourth pairs of spinal nerves that the ven- 

 trals receive their supply ; the third specially 

 supplying the muscles of the pelvis, to which 

 likewise the fourth give some branches, but the 

 latter is more particularly distributed to the fin- 

 rays. The muscles of this fin likewise derive 

 some filaments from the fifth pair of spinal 

 nerves. 



In the jugular division of Malacopteryginous 

 Fishes, in which the ventral fins are attached 



