SWIMMING-BLADDER. 645 



(1758.) In connexion 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 required depth, without exertion. 



(1759.) The apparatus given for this purpose is called the swimming- 

 bladder, and consists of a reservoir of air (fig. 317, 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 inter- 

 nally with a thin and delicate membrane. 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 ; sometimes, as in the Cyprinidae, it is divided 

 into two portions, one anterior and the other posterior, by a deep cen- 

 tral 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 as the compressed 

 ' air is again permitted to expand, the creature becoming more buoyant 

 rises towards the surface. 



(1760.) In the Perch, and many other fishes, this organ is entirely 

 closed, so that there is no escape for the contained air ; and in such it 

 has been found that if they are suddenly brought up by means of a line 

 from any great depth, the gas, being no longer compressed by the weight 

 of the column of water above, and having no exit, bursts the swimming- 

 bladder, and sometimes distends the abdomen to such an extent, that it 

 pushes the stomach and oesophagus into the fish's mouth. 



(1761.) In other cases, however, a provision is made apparently with 

 the view of obviating such an accident, and a kind of safety-valve pro- 

 vided through which the air may be permitted to escape : thus, in the 

 Carp, a tube communicates between the interior of the air-bladder and 

 the oesophagus, and in the Herring a similar communication is met with 

 between this organ arid the stomach. 



(1762.) The gas which fills the air-bladder has been found in many 



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 cer- 

 tain sauroid fishes, as the Lepidosteus, which likewise possesses the opercular bones. 

 Thirdly, that they are parts of the dermal skeleton in short, scales modified in sub- 

 serviency to the breathing function, 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. 



