(1818). This branch is superficial until it reaches the little muscles that move the fin. It has sometimes other branches, equally superficial, which descend to the anterior parts of the muscles of the trunk above the pectoral fins; and others which run as far as the anal fin, where they form a longitudinal nerve similar to that of the back.

(1819). The seventh pair of cerebral nerves (fig. 319, s s), in fishes as in all other Vertebrata, is devoted to the organ of hearing, and brings to the sensorium the impressions of sound.

(1820). The sense of hearing in these creatures must necessarily be very imperfect: they have neither an external ear nor a tympanic cavity, and consequently are entirely destitute of a membrana tympani and of the ossicles of hearing; they have neither Eustachian tube nor fenestra ovalis; the labyrinth alone, and that more simple in its composition than the labyrinth of the human ear, is all that the anatomist meets with in this first appearance of an auditory apparatus among the Vertebrate classes.

(1821). The accompanying figure (fig. 323) represents the ear of a very large fish, the Lophius piscatorius; and the student will have little difficulty in at once recognizing all the parts of which it consists. The soft parts of this simple ear are not enclosed in bony canals, as in the human subject, but the membranous labyrinth is lodged in a wide cavity on each side of the cranium; so that little dissection is necessary to expose the entire organ, which is surrounded on all sides with the same kind of oily or mucilaginous fluid which fills up the wide interspace that exists between the brain and the dura mater lining the inner surface of the skull.

Auditory apparatus of the Skate.

Fig. 323. Auditory apparatus of the Skate.

(1822). As in all other Vertebrata, there are three semicircular canals, disposed nearly as in the human ear, and each dilated in like manner into an ampulla which receives the filaments of the acoustic nerve. Two of the semicircular canals coalesce before they open into the vestibule, so that there are only five orifices whereby the three semicircular canals communicate with the vestibular cavity.

(1823). The membranous vestibule (supported in the figure by two pins) is of variable shape, and its walls are very delicate. Its cavity, as well as the interior of the semicircular canals, is filled with a transparent glairy fluid; and it moreover encloses certain hard bodies (otoliths), generally three in number, suspended by delicate filaments in its interior.

(1824). The otoliths of osseous fishes are of a stony hardness, resembling shell, and their structure is nothing at all like that of bone.

(1825). Their shape varies in different species, but, nevertheless, is so constantly the same in fishes of the same kind, that the forms of these pieces might be employed as an important zoological character.

(1826). In the cartilaginous fishes the otoliths are quite soft, resembling starch: in both classes they are composed principally of chalk, and effervesce strongly when dissolved in acids.

(1827). The auditory nerve gives a filament to each of the semicircular canals, which penetrates into the ampulla of the canal to which it is destined, and there spreads out; but the larger portion of the nerve is distributed over the vestibular sacculus, where it forms a beautiful network.

(1828). There is no cochlea, although some writers imagine that they can distinguish a rudiment of this part of the ear in a slight projection from the walls of the vestibule.

(1829). The ears of fishes, therefore, are much less perfect than those of other Vertebrata*: deprived of tympanum, of ossicles, and of Eustachian tube, they can scarcely receive the impressions produced by the vibrations of the ambient element, except by those vibrations being communicated through the cranium; and moreover, the membranous labyrinth not being enclosed in bone, the skull can only transmit these movements in a very feeble and imperfect manner. The absence of a cochlea would go far to prove that the ear of fishes cannot appreciate the differences of tones. All that it offers to the physiologist is a membranous apparatus endowed with great sensibility, in which the nervous filaments distributed in the ampullae of the semicircular canals must necessarily partake of all the movements of the fluid in which they are plunged, and where those appropriated to the vestibule must be still more strongly agitated by the shocks that these movements give to the otoliths contained in its cavities.

(1830). It is probable, therefore, that fishes hear, that noise produces in them a powerful sensation, but that they cannot distinguish or appreciate differences of tone, as the higher animals are enabled to do.

(1831). The nerves composing the eighth pair preside over the same functions in all the Vertebrata. The glosso-pharyngeal sends twigs to the first branchial arch, the fauces, and the tongue. The nervus vagus (fig. 319, t) supplies the three posterior branchiae and the lower part of the pharynx; it is then continued along the oesophagus to the stomach, where it terminates: it thus presides over the same functions in all the Vertebrate classes; and it is not a little interesting to see it even in fishes distributed to the organs of respiration, notwithstanding the peculiarity of their structure and position. In these creatures, however, it likewise furnishes nerves to other parts of the body, especially a long branch, which generally runs in the substance of the lateral muscles of the trunk, communicating with the spinal nerves and giving off filaments to the skin - an arrangement the physiology of which is not as yet understood. The next pair of cerebral nerves in the animals under consideration would seem to represent the spinal recurrent of the human subject; it supplies the swimming-bladder and the muscles of the shoulder.