SECRETION AND ABSORPTION OF GAS, ETC. 121 



Moreau, 1 whose work, however, does not seem by any 

 means so well known as it ought to be. He proved that 

 the current ideas about the mode of action of the air-bladder 

 were entirely erroneous, and that a fish in reality makes 

 no use of its muscles in regulating the volume of its air- 

 bladder. When the fish descends, the air in the bladder is 

 compressed and the specific gravity of the body increases, 

 so that it naturally tends to sink farther and farther, and if 

 it got too far would sink to the bottom. Conversely, if the 

 fish ascends its air-bladder expands, and if it goes too far it 

 is helplessly carried to the surface, unless it possesses a 

 permeable air-duct or the air-bladder bursts outwards. Thus 



Fig. 1. 



the air-bladder of a fish tends to make it behave exactly like 

 the toy known as the " Cartesian Diver". From this point 

 of view it would seem as if an air-bladder must be a source 

 of great inconvenience to a fish, exposing it to a constant 

 danger of being carried helplessly to the surface or to the 

 bottom. Its position at the best is evidently one of un- 

 stable equibibrium, which the smallest movement upwards 

 or downwards could disturb. 



Moreau demonstrated the facts just mentioned by means 

 of several experiments, one or two of which may be de- 

 scribed. He placed the fish in a large and tall glass vessel 

 completely filled with water (Fig. 1). Into this vessel 



1 Me moires de Physiologie, Paris, 1877. 



