610 



A HISTORY OF 



to get more air ; the bubbles on the surface 

 begin to disappear ; the belly, that was before 

 swollen, will then fall of a sudden ; and the 

 animal sinks expiring and convulsed at the 

 bottom. 



So very necessary is air to all animals, but 

 particularly to fish, that, as was said, they can 

 live but a few minutes without it ; yet nothing 

 is more difficult to be accounted for than the 

 manner in which they obtain this necessary 

 supply. Those who have seen a fish in the 

 water, must remember the motion of its lips 

 and its gills, or at least of the bones on each 

 side that cover them. This motion in the ani- 

 mal is, without doubt, analogous to our breath- 

 ing ; but it is not air, but water, that the fish 

 actually sticks in and spouts out through the 

 gills at every motion. The manner of its 

 breathing is thus : the fish first takes in a 



auantity of water by the mouth, which is 

 riven to the gills ; these close and keep the 

 water so swallowed from returning by the 

 mouth ; while the bony covering of the gills 

 prevents k from going through them, until 

 the animal lias drawn the proper quantity of 

 air from the body of water thus imprisoned : 

 then the bony covers open, and give it a free 

 passage ; by which means also the gills again 

 are opened, and admit a fresh quantity of 

 water. Should the fish be prevented from the 

 free play of its gills, or should the bony covers 

 be kept from moving, by a string tied round 

 them, the animal would soon fall into convul- 

 sions, 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 con- 

 taining it ; the stomach being the chief cavity 

 within the body, is too much filled with aliment 

 for that purpose. 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 purposes ; but that our philosophers 

 have long destined to a very different use. 

 The use universallyassigned to the air-bladder, 

 is the enabling the fish to rise or sink in the 

 water at pleasure, as that is dilated or com- 

 pressed. The use assigned by the ancients 

 fur it was to come in aid of the lungs, and to 

 remain as a kind of store- house of air to sup- 

 ply the animal in its necessities. I own my 



attachment to this last opinion ; but let us ex- 

 hibit both with their proper share of evidence, 

 and the reader must be left to determine. 



The air-bladder is described as a bag filled 

 with air, sometimes composed of one, some- 

 times 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 rais- 

 ing 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 extraordinary 

 manner, till the bladder bursts, and then the 

 fish sinks, and ever after continues to crawl at 

 the bottom. On another occasion, 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 after. From 

 thence 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 enable 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 bring thus rendered slimi.fer and 

 heavier, consequently sinks to the bottom. 



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 v\\] a 

 mouse or a frog ; and these we know to have 

 no air-bladders. A carp will rise to the sur- 

 face ; 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 experi- 

 ment ; 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 sound. 

 The fish after the experiment, I grant, con- 

 tinues to creep at the bottom ; and so will all 

 fish that arc sick and wounded, which must 

 be the case with this aftor such an operation. 

 Thus these facts prove nothing, but that when 

 the fish is killed in an air pump the air-blad- 

 der is found exhausted, and that it will natu- 

 rally and necessarily be ; for the drain of air 

 by which the fish is supplied in the natural 



