ioO HISTOKY OF 



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 manne. 

 in which they obtain this necessary supply. Those who have 

 seen a fish in the water must remember the motion of its lijis 

 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 ac- 

 tually sucks in and spouts out through the gills at every motion. 

 The maimer 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 pro- 

 per 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 respira- 

 tion 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 philosophers 

 have long destined to a very different use. The use universally 

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

 sink ui 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 store-house of air to sup- 

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

 last opinion ; but let ns exhibit both with their proper share of 

 evidence, and the reader must be left to determine. 



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



