X TELEOSTEAN FISHES 357 



produced in its specific gravity by changes of level and consequent 

 changes in pressure. It is obvious that if a fish swims downwards it 

 becomes subjected to greater and greater pressure due to the super- 

 incumbent water ; the gas in its air-bladder will be more and more com- 

 pressed by this pressure, and the specific gravity of the fish will increase. 

 If it started by being of exactly the same specific gravity as the water, 

 it will become relatively heavier and heavier as it swims downwards and 

 will consequently tend to sink. Conversely if it swims upwards the gas 

 in its air-bladder under the diminishing pressure will expand more and 

 more, and it will tend to be carried helplessly up to the surface. It is 

 the function of the air-bladder to counteract these inconveniences and 

 dangers by keeping the body of the fish precisely at the specific gravity 

 of the surrounding water, so that it floats at one level without requiring 

 to expend muscular energy in combating a tendency either to float 

 upwards or to sink downwards. This is achieved not by muscular 

 action but by alteration of the actual quantity of gas in the air-bladder. 

 The compression due to increased pressure is met by increasing the 

 amount of gas ; the expansion due to diminished pressure is met by re- 

 ducing the amount of gas. The first-mentioned process, the increase 

 in the amount of gas, is clearly brought about by the activity of the wall 

 of the air-bladder, for it takes place in physostomatous fishes which are 

 prevented from taking in air at the surface, and in physoclistic fishes 

 where the air-bladder has no longer any opening. It might be, and 

 actually was at one time, supposed that the additional gas is provided 

 by a simple process of diffusion from the blood circulating in the wall of 

 the organ. That this is not the case, however, became apparent when 

 analyses were made of the gas in the air-bladder. This is found to 

 consist of the ordinary gases of the atmosphere but by no means in their 

 ordinary proportion. In fishes from considerable depths in the sea there 

 is usually a very high proportion of oxygen — ranging it may be up to 

 more than 90 per cent. Now the ordinary processes of diffusion could 

 not bring about a concentration of oxygen higher than that in the blood 

 itself. In fact were gas containing this high percentage of oxygen in a 

 cavity of the body subject to the ordinary laws of diffusion, the tendency 

 would be rather for it to diffuse away until the oxygen pressure was no 

 greater than that of the oxygen in the blood and other fluids of the body. 

 The process at work then must be no mere physical process of diffusion 

 but something deflnitely vital which can force gas into the cavity of the 

 swim-bladder in spite of a high pressure of the same gas already within 

 the cavity. As a matter of fact the process seems to be entirely analogous 

 to the process of secretion in a gland : the air-bladder is in a sense a 



