FISHES. 203 



bulk of water. If the specific gravity should he 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 e.\ertion, is exhibited by a singular and 

 eflfectual piece of mechanism, provided apparently for this 

 purpose. It is a membranous 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 con- 

 sists of two or more membraneous bags, with small connect- 

 ing apertures, or with the divisions quite distinct, or with 

 ])rolongations 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 lunsr of the air-breathinii: vertebrate animals. It is 

 brought as we have seen (p. 201), into connexion with the 

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

 fishes it is subservient to the production of sounds, which are 

 caused by the air passing from the air-bladder, by means of an 

 air-duct, into the gullet (asophagm). 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 pro- 

 tection against the too sudden expansion of the gas, when the 

 fish rises to the surface. t 



When we besrin 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 fiat 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 lied Mullet, which has no swimming- 



* Lectures, p. 227. 



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

 the constituent's of atmospheric air in varying proportions. No htjdrogen has 

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



