23f A HISTORY OF 



from moving, by a string tied round them, the animal would soon fall 

 l*io convulsions, and die in a few minutes. 



But though this be the general method of explaining respiration 

 in fishes, the difficulty remains to know what is done with this air 

 which the fish in this manner separates from the water. There seems 

 no receptacle for containing it ; the stomach being the chief 

 cavity within the body, is too much filled with aliment for that pur- 

 pose. There is indeed a cavity, and that a pretty large one, I mean 

 the air-bladder or swim, which may serve to contain it for vital pur- 

 poses ; but that our philosophers have long destined to a very differ- 

 ent use. The use universally assigned to the air-bladder, is the ena- 

 bling the fish to rise or sink in the water at pleasure, as that is dilated 

 or compressed. The use assigned by the ancients for it was to come 

 in aid of the lungs, and to remain as a kind of storehouse of air to 

 supply the animal in its necessities. I own my attachment to this 

 last opinion ; but let us exhibit both with their proper share of evi- 

 dence, and the reader must be left to determine. 



The air-bladder is described as a bag filled with air, sometimes 

 composed of one, sometimes of two, and sometimes of three divisions, 

 situated towards the back of the fish, and opening into the maw or the 

 gullet. Those who contend that this bag is designed for raising or 

 depressing the fish in the water, build upon the following experiment. 

 A carp being put into the air-pump, and the air exhausted, the bladder 

 is said to expand itself to such a degree, that the fish swells in an ex- 

 traordinary manner, till the bladder bursts, and then the fish sinks, 

 and ever after continues to crawl at the bottom. On another occa- 

 sion, the air-bladder was pricked and wounded, which let out its air ; 

 upon which the fish sunk to the bottom, and was not seen to rise af- 

 ter. From hence it is inferred, that the use of the air-bladder must 

 be by swelling, at the will of the animal, thus to increase the surface 

 of the fish's body, and thence diminishing its specific gravity, to en- 

 able it to rise to the top of the water, and keep there at pleasure. 

 On the contrary, when the fish wants to descend, it is, say they, but 

 to exhaust this bladder of its air ; and the fish being thus rendered 

 slimmer and heavier, consequently sinks to the boltom. 



Such is the account given of the use of the air-bladder ; no part of 

 which seems to me well supported. In the first place, though nothing 

 is more certain, than that a carp put into the air-pump will swell, yet 

 so will a mouse or a frog ; and these we know to have no air-bladders. 

 A carp will rise to the surface : but so will all fish that want air, 

 whether they have an air-bladder or not. The air-bladder is said to 

 burst in the experiment ; but that I deny. The air-bladder is indeed 

 found empty, but it has suffered no laceration, and may be distended 

 by being blown into like any other bladder that is found. The fish, af- 

 ter the experiment, I grant, continues to creep at the bottom ; and so 

 will all fish that are sick and wounded, which must be the case with 

 this after such an operation. Thus these facts prove nothing, but that 

 when the fish is killed in an air-pump the air-bladder is found ex- 

 hausted, and that it will naturally and necessarily be ; for the drain 

 of air by which the fish is supplied in the natural way will necessarily 

 oblige it to make use of all its hidden stores ; and, as there is a com- 

 munication between the gullet and the air-bladder, the air which the 



