126 SCIENCE PROGRESS. 



oxygen than air. It would seem therefore that pure, or 

 nearly pure, oxygen is secreted into the bladder. A similar 

 result is obtained by sinking a surface fish into deep water 

 in a cage. As much as 90 per cent, of oxygen may be 

 present in the gas from the bladder. 



It is generally assumed that the passage of gases 

 throughout the body, and between the body and its environ- 

 ment, is due simply to diffusion. This theory is probably 

 perfectly satisfactory for many cases, but does not account 

 for the facts connected with the swimming-bladder. Sup- 

 posing the bladder to have been originally filled with air, 

 which had passed in, it might be, through the duct when 

 the fish was young, what would happen to this air on 

 the diffusion theory? In the first place its oxygen per- 

 centage would never increase. The amounts of oxygen 

 and nitrogen dissolved in sea water depend on the tensions 

 or partial pressures of each of these gases in the atmo- 

 sphere, and on their co-efficients of solubility. Sea water 

 is known to be saturated with nitrogen, and more or less 

 saturated with oxygen at the pressures which these gases 

 respectively exercise in air. Supposing some of this gas to 

 come in contact with an atmosphere of gas in which the 

 partial pressure of nitrogen or oxygen was less than in air, 

 then nitrogen or oxygen would be given off from the 

 water until a state of equilibrium was again established. 

 Conversely, if the partial pressure of nitrogen or oxygen 

 in the gas was greater than in air the water would take up 

 these gases. The air and the water have the same gas-ten- 

 sions when the equilibrium is established. Two liquids, such 

 as sea water and the blood passing through the gills, will by 

 simple diffusion tend to assume the same oxygen, nitrogen 

 and carbonic acid tensions. On the diffusion theory the 

 blood of course cannot have a higher oxygen tension than 

 the air, hence it cannot give up oxygen to the air-bladder 

 if the latter already contains oxygen at atmospheric tension. 

 Actually, however, the oxygen tension o^ blood passing 

 through a tissue such as the wall of the air-bladder will be 

 less than that of air, owing to the using up of oxygen for 

 respiratory purposes. Hence the blood will absorb oxygen 



