PISCES FISHES. 509 



of the Sturgeon ossified and converted into bone, than to be com- 

 pelled to have recourse to the bold speculations of the French 

 anatomist regarding the real nature of these opercular portions of 

 a fish's skeleton.* 



(543.) In connection with the locomotive organs we must 

 here notice one of the most elegant contrivances met with in the 

 whole range of animated nature, by which the generality of fishes 

 are enabled to ascend towards the surface, or to sink to any re- 

 quired depth without exertion. 



The apparatus given for this purpose is called the swimming- 

 bladder, and consists of a reservoir of air (Jig. 227, p) placed 

 beneath the spine; in which position it is firmly bound down by 

 the peritoneum. The outer coat of this bladder is very strong, and 

 composed of a peculiar fibrous substance from which isinglass is 

 obtained, but it is lined internally with a thin and delicate mem- 

 brane. The shape of the swimming-bladder varies considerably in 

 different tribes. In the Perch it is a simple cylinder closed at both 

 extremities : sometimes it gives off branched appendages ; some- 

 times, as in the Cyprinid&i it is divided into two portions, one 

 anterior and the other posterior, by a deep central constriction ; 

 but, whatever its shape, its office is the same, namely, to alter the 

 specific gravity of the fish, and thus to cause it to rise or sink in 

 the medium it inhabits. By simply compressing this bladder by 

 approximating the walls of the abdomen, or occasionally by means 

 of a muscular apparatus provided for the purpose, upon a principle 

 with which every one is familiar, the fish sinks in proportion to the 

 degree of pressure to which the contained air is subjected ; and, 



* The different opinions on the nature or homology of the opercular bones may be 

 reduced to two principles : first, that they are modifications of parts of the ordinary 

 skeleton ; secondly, that they are superadded bones peculiar to fishes : the latter view 

 is that taken by Cuvier. According to the former, which is the more philosophical mode 

 of considering them, three opinions have been offered ; the first by Spix and Geoffrey, 

 that they are gigantic representatives of the ossicles of the ear, otherwise absent in the 

 skeleton of fishes, this view has been adopted by Professor Grant ; secondly, that they 

 are dismemberments of the lower jaw, which by the detachment of the opercular bones 

 from the ramus is rendered more simple in its composition than in reptiles, a view 

 proposed by M. de Blainville and temporarily adopted by Bojanus and Oken, but refuted 

 by the complicated structure of the lower jaw in certain sauroid fishes, as the Lepi- 

 dosteus, which likewise possesses the opercular bones; thirdly, that they are parts of 

 the dermal skeleton, in short, scales modified in subserviency to the breathing func- 

 tion ; an opinion first proposed by Professor Owen, in his Lectures on Comparative 

 Anatomy at St. Bartholomew's Hospital in 1835, and which is the view here adopted. 



