RESPIRATION 209 



downwards from the position of equilibrium will similarly tend 

 to make the animal sink with increasing velocity to the bottom. 

 When fishes are stunned by an explosion under water, about half 

 of them float to the top, and the other half sink to the bottom. 

 One has only to place a goldfish in a large and tall bottle of 

 water provided with a perforated cork through which a thick 

 walled tube containing water passes to another small bottle of 

 water, in order to see how the fish deals with the situation. If the 

 pressure in the large bottle is raised by raising the small bottle 

 the fish will at first begin to sink, but will immediately turn its 

 nose upwards and swim upwards, so as to reestablish its position 

 of unstable equilibrium; and conversely if the large bottle be 

 lowered. It was formerly believed that a fish compresses or relaxes 

 its swim bladder when it wishes to go downwards or upwards. 

 That this is not the case was shown by Moreau^ in a series of 

 beautiful experiments. A fish is really confined temporarily to 

 about a certain depth by its swim bladder ; for if any cause tends 

 to make it leave this depth the animal's response to the stimulus 

 of expansion or contraction of its swim bladder soon brings it 

 back to its proper depth. 



The goldfish has an open duct to its swim bladder, so if the 

 pressure is greatly diminished, as by connecting the large bottle 

 to a filter pump, the air of the swim bladder comes bubbling out 

 of the animal's mouth. If the pressure is now restored to normal 

 the animal sinks to the bottom, and after a few fruitless efforts 

 to swim upwards lies helpless on its side. If it is left there for some 

 time, however, it gradually becomes more buoyant, and after a 

 certain number of hours it will be swimming about as usual, with 

 its swim bladder full of gas. If a fish has a closed swim bladder, 

 and the gas from this is removed by means of a hypodermic 

 syringe, the fish also sinks at first, but soon refills its swim bladder 

 with gas. How is this gas produced, and what is it? It cannot have 

 been swallowed as air, as the fish has been lying in water at the 

 bottom all the time, or has a closed swim bladder. This brings 

 us to gas secretion. 



About the beginning of last century the eminent French physi- 

 cist Biot was engaged in survey work in the Mediterranean, and 

 was attracted by the observation that fishes caught with a line at 

 great depths come to the surface and lie helpless with their swim 

 bladders distended with gas and sometimes projecting out through 

 the mouth. He determined to analyze the gas, and having intro- 



* Moreau, Memoires de Physiologic, Paris, 1877. 



