FUNCTIONS OF SWIMBLADDER OF FISHES. 99 



alveolar air simply play a passive part, and physical diffusion 

 explains the respiratory exchange. However, this view has not 

 gone unchallenged. 



Haldane and Smith (1896) claims that the tension of the oxy- 

 gen in the arterial blood may be higher than the pressure of oxy- 

 gen in the alveoli. According to the physical theory of respi- 

 ration, if a permeable membrane separates two volumes of any 

 gas, or two solutions of any gas at different pressures, the mole- 

 cules of the gas will pass through the membrane in both directions 

 until the pressure is equal on both sides; i.e., gas diffuses from a 

 point of high rension to one of low tension. Haldane maintains 

 that a secretory activity is associated with the function of the 

 lungs, which produces a higher tension of oxygen in the arterial 

 blood than in the alveolar air. 



The analogy has many times been drawn between the gas inter- 

 change in the lungs and in the swimbladder of the fish. Bohr 

 (1894) punctured the swimbladder and removed gas. He found 

 that if the fish was then left in the water, the gas was rapidly 

 replaced, and, when he tapped the swimbladder a second time, 

 that the percentage of oxygen had greatly increased. Oxygen 

 may amount to between 60 and 80 per cent of the total gas after 

 such an experiment. Bohr found that this reaccumulation did 

 not take place if both vagi were cut and he ascribed it therefore to 

 direct secretory activity on the part of the swimbladder. Bohr 

 is inclined to endow the vagus nerves of the higher vertebrates, 

 including mammals, with an analagous regulatory influence on 

 the gaseous exchange in the lungs. 



Starling (1920) strongly maintains that the lungs cannot be 

 considered analagous to the swimbladder. He says there is no 

 likeness between the thick secreting cells of the "red gland" 

 which is apparently the gas-secreting part of the swimbladder, 

 and the thin structureless plates which separate the capillaries 

 of the lungs from the alveolar air. 



It was considered necessary to repeat some of Bohr's work 

 during the present experiments, using fresh-water fishes. Yellow 

 perch were used for the first series. Gas samples were obtained 

 by methods already described (p. ooo). After the first sample 

 was taken the fishes were placed in the same tank in which they 

 had been accustomed to live. No changes in the amounts of 



