246 



HISTORY OF FISHES. 



So very necessary is air to all animals, but j 

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

 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 ne- j 

 cessary 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 animal is, without doubt, analogous to our 

 breathing ; but it is not air, but water, that 

 the fish actually sucks in and spouts out 

 through the gills at every motion. The man- 

 ner of its breathing is thus : the fish first takes 

 in a quantity of water by the mouth, which is 

 driven to the gills ; these close and keep the 

 water so swallowed from returning by the 

 mouth; while the bony covering of the gills 

 prevents it from going through them, until the 

 animal has drawn the proper quantity of air 

 from the body of water thus imprisoned : then 

 the bony-covers open, and give it a free pas- 

 sage : 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 to be no receptacle 

 for containing 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 phi- 

 losophers have long destined to a very differ- 

 ent use. The use universally assigned to the 

 air-bladder, is the enabling the fish to rise or 

 sink in the water at pleasure, as that is dilated 

 or compressed. The use assigned by the an- j 

 cients lor it was to come in aid of the lungs, 

 and to remain as a kind of store-house 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, arid 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 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 j 

 put into the air-pump, and the air exhaus- ! 

 ted, the bladder is said to expand itself to I 

 such a degree, that the fish swells in an ex- I 



traordinary manner, till the bladder bursts, 

 and then the fish sinks, and ever after conti 

 nues to crawl at the bottom. On another 

 occasion, the air-bladder was pricked arid 

 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 swell- 

 ing, at the will of the animal, thus to increase 

 the surface of the fish's body, and thence di- 

 minishing 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 ex- 

 haust this bladder of its air; and the fish 

 being thus rendered slimmer 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 will 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, whe- 

 ther 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 sound. The fish 

 after 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 exhausted, 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 commu- 

 nication between the gullet and the air-blad- 

 der, the air which the latter contains will thus 

 be obviously drawn away. But still farther, 

 how comes the air-bladder, according to their 

 hypothesis, to swell under the experiment of 

 the air-pump ? What is it that closes the 

 aperture of that organ in such a mannei as 

 at last to burst it? or what necessity has the 

 fish for dilating it to that violent degree? At 

 most, it only wants to rise to the surface ; and 

 that the fish can easily do without so great 

 a distention of the air-bladder. Indeed it 

 should rather seem that the more the air was 

 wanted without, the less necessity there was 

 for its being uselessly accumulated within ; 

 and, to make the modern system consistent, 

 the fish under the air-pump, instead of per- 

 mitting its bladder to burst, would readily 

 give up its contents ; which, upon their sup- 

 position, all can do at pleasure. 



