PISCES. 



1005 



The tissue of which they are composed is similar 

 to that which forms the membranous vestibule 

 and saccus; it is, however, a tissue sui generis, 

 being neither exactly comparable to cartilage, 

 nor tendon, nor cellular membrane. It is pel- 

 lucid, and when emptied of the enclosed fluid, 

 inelastic, but flexible and easily torn. Its 

 thickness is greater than that of the vestibule or 

 of the sac of the otolithe ; but the ampullae 

 seem thicker than the rest, for when wounded 

 and their contents allowed to escape they still 

 retain their form and expansion. 



The membranous labyrinth is filled with a 

 limpid fluid. 



Auditory nerves.- The labyrinth of the ear 

 in Fishes receives its nerves from two sources,* 

 1st, from the auditory nerve, properly so called, 

 which is distributed to the membranous vesti- 

 bule, and to the ampullae of the anterior and 

 external semicircular canals ; 2ndly, from the 

 "accessory auditory nerve," which, in most 

 instances, seems to arise not from the brain but 

 from the trigeminal or the vagus nerve, and 

 supplies the ampulla of the posterior semicir- 

 cular canal and the saccus. 



Ear of plagiostome cartilaginous Fishes. 

 In the Skate are two canals, regarded by Monro 

 as representing the meatus auditorius externus. 

 The orifices of these are situated at the upper 

 and back part of the head at a short distance 

 from the junction of the skull with the first cer- 

 vical vertebra, the opening of each being large 

 enough to admit the end of a probe. Each of 

 these orifices leads to a winding canal about 

 two lines in diameter, which, after describing 

 more than three-fourths of a circle, may be 

 traced into the membranous vestibule of the 

 ear. This canal is generally found filled with 

 a white viscid matter. The vestibule is a large 

 sac containing a very viscid pellucid humour, 

 in consistence like the white of an egg, in 

 which is suspended a soft cretaceous substance. 

 To the anterior part of the large sac there is 

 a smaller compartment communicating with the 

 former by a narrow passage, which is likewise 

 filled with glairy fluid, and, posteriorly, there is 

 a third very small sacculus, similarly distended, 

 in both of which cretaceous matter is found. 



The remaining portion of the internal ear 

 consists of three canals, analogous to the semi- 

 circular canals of the higher Vertebrata, but 

 which here rather deserve the name of circular, 

 seeing that each forms a complete circle; of 

 these the anterior and the middle are joined 

 together at their commencement by the wide in- 

 tercommunicating branch which opens through 

 the intervention of a small membranous tube 

 into the anterior small sac of the vestibule. 

 The third or posterior canal communicates with 

 the large sac of the vestibule by means of a 

 wide canal, but has no direct communication 

 with either of the others. 



Each circular canal has a dilated portion or 

 ampulla near one of its extremities, and is filled 

 with a pellucid viscid fluid. They are all con- 

 tained in cartilaginous tubes excavated in the 

 cartilaginous substance of the cranium, but 



* Weber, loco cit. 



much wider than the membranous canals them- 

 selves, the latter being suspended in a fluid 

 interposed between them and the perichondrial 

 lining of the cartilaginous passages, to which 

 they are fixed by a delicate cellulosity, in which 

 slender vessels and very minute nerves are 

 visible. 



The auditory nerve on entering the ear di- 

 vides into several branches. Of these the prin- 

 cipal spreads out upon the inferior aspect of 

 the great sac of the vestibule, where it forms a 

 rich plexus; a similar but smaller plexus is 

 formed upon the smaller anterior sac commu- 

 nicating with the vestibule, while the other 

 branches are appropriated to the semicircular 

 canals, on the ampullae of which they would 

 seem to be exclusively distributed ; at least 

 after forming a very beautiful expansion upon 

 the dilated portion of the canal, it is impossible, 

 owing perhaps to their very minute size, to 

 trace them any further over its cylindrical part. 



Generative system. One of the most re- 

 markable circumstances connected with the 

 history of the finny tribes is their extreme 

 fertility, which, compared with that of the 

 higher Vertebrata, is truly prodigious. A cod- 

 fish has been calculated to produce 9,000,000 

 of eggs in a single season, and innumerable 

 races of the osteopteryginous Fishes exhibit 

 powers of reproduction equally extraordinary. 

 To imagine that this exuberant fecundity is 

 destined merely for the purpose of perpetuating 

 the species would evidently be preposterous, 

 and we are necessarily led to look for other rea- 

 sons explanatory of such teeming births. There 

 is this leading difference between the terrestrial 

 and aquatic domains of animated nature the 

 earth is inhabited only at its surface, and the 

 vegetable banquet which is there spread out in 

 such rich abundance is sufficient to afford the 

 means of subsistence to all earth's progeny. 

 But the sea, throughout all its depth, at every 

 altitude which man has been able to explore, 

 is peopled with innumerable races of voracious 

 beings, all of which are necessarily dependent 

 for their existence upon a supply of animal 

 food, which must consequently be distributed 

 as widely as the waves of ocean are diffused. 

 It is to supply this great stock of living pro- 

 vender that the Sponges and the Polyps and 

 all the humbler marine forms of existence are 

 continually pouring forth their multitudinous 

 germs, and it is for the purpose of adding to 

 this enormous store that the majority of the 

 osseous Fishes are so inordinately prolific. 



From these considerations we perceive at 

 once a reason for the extraordinary apathy and 

 total absence of parental affection which forms 

 so conspicuous a feature in the character of the 

 whole race, and it is by no means a subject 

 devoid of interest to observe how gradually 

 the ties between parent and offspring are drawn 

 closer and closer as we ascend from these 

 humblest members of the Vertebrata and arrive 

 at progressively increasing intelligence as we 

 advance from class to class. 



The generative apparatus of Fishes, as we 

 have pointed out in a preceding article, (GE- 

 NERATION, ORGANS OF, Comp. Anat.,) pre- 



