FISHES. 2 1 9 



bulk of water. If the specific gravity should be increased, 

 the fish would necessarily descend, without any muscular exer- 

 tion; or, if diminished, the fish would become lighter than the 

 water, and would, therefore, rise to the surface. A beautiful 

 arrangement, by which the fish can thus rise or sink at plea- 

 sure, and without exertion, is exhibited by a singular and 

 effectual piece of mechanism, provided apparently for this 

 purpose. It is a membraneous bag, placed at the lower side 

 of the spinal column, and known as the " swim-bladder " or 

 " air-bladder." In the Cod-fish it is the part which is called 

 the ' ' sound. ' ' It differs much in form, and sometimes consists 

 of two or more membraneous bags, with small connecting 

 apertures, or with the divisions quite distinct, or with pro- 

 longations from the sides or ends.* But whatever be the 

 form, the principal use seems to be the same namely, that of 

 enabling the fish to regulate the specific gravity of its body. 



Professor Owen regards it as the representative in fishes 

 of the true lung of the air-breathing vertebrate animals. 

 It is brought, as we have seen (p. 217), into connexion with 

 the chamber or labyrinth of the organ of hearing; and in a 

 few fishes, such as the Gurnard, it is subservient to the pro- 

 duction of sounds, which are caused by the air passing from 

 the air-bladder, by means of an air-duct, into the gullet 

 (oesophagus). It appears also to act, in some cases, as a 

 safety-valve against high-pressure, when the fish sinks to 

 great depths, and to a limited extent as a protection against 

 the too sudden expansion of the gas, when the fish rises to 

 the surface.! 



When we begin to examine to what extent this mechanism 

 prevails among fishes, we find it is by no means universal. 

 It is not observed in the Plaice, the Turbot, the Sole, and 

 other flat-fishes; and as these different species live near the 

 bottom of the water, we are at first inclined to say it is not 

 given to them for that reason, but that it is given to those 

 which are in the habit of rising and sinking. A little further 

 examination, however, shows that we are mistaken. Eels, 

 which live near the ground, have the swimming bladder well 

 developed; while the Red Mullet, which has no swimming 



* Lectures, p. 227. 



f The gas in the air-bladder is found to consist of nitrogen and oxygen, 

 the constituents of atmospheric air, in varying proportions. No hydrogen 

 has ever been detected Owen's Lectures, p. 277. 



