SECRETION AND ABSORPTION OF GAS IN 

 THE SWIMMING-BLADDER AND LUNGS. 



PART I.— SWIMMING-BLADDER. 



IN the present paper I propose to give a short account 

 of what has been ascertained with regard to the con- 

 ditions which determine separation and absorption of gas 

 by the swimming- or air-bladder. The subject is one of 

 considerable interest, since it seems to lead up very directly 

 to certain questions of fundamental importance in biology. 



The air-bladder, as is well known, is an elongated sac 

 containing gas, and usually lies dorsal to the alimentary canal, 

 of which it is an out-growth. It is present in most, but 

 not in all, fishes. The duct communicating with the ali- 

 mentary canal is in some species permeable, and in others 

 completely closed. 



It has been generally assumed for long that the chief 

 function of the air-bladder is to enable the animal to increase 

 or diminish its specific gravity, and thus alter the depth at 

 which it swims. Borelli in his famous book, De Motu Ani- 

 malium, says that when it wishes to go down, the fish, by 

 means of its muscles, compresses the air-bladder, and thus 

 diminishes the volume and correspondingly increases the 

 specific gravity of its body. When the fish wishes to rise it 

 relaxes its muscles, and thus diminishes the specific gravity. 



Another theory which deserves mention is that of Dela- 

 roche, 1 who supposed that the muscles which compress the 

 swimming-bladder are in a state of tonic contraction, regu- 

 lated according to the depth at which the animal wishes to 

 remain. When the animal rises or goes down the contrac- 

 tion becomes more, or less, vigorous, so that the specific 

 gravity is kept constant. The function of the air-bladder 

 is thus to enable the animal to remain at any desired depth 

 in the water. 



The whole subject of the functions of the air-bladder 

 was carefully investigated about twenty years ago by 



1 Annates du Museum Shistoire naturelle, 1809. 



