466 THE EXIT OF CARBONIC ACID. [BOOK n. 



The partial pressure of the oxygen in the gas in the swim-blad- 

 der of fishes for instance far exceeds that of the fishes' blood ; 

 and if the gas be drawn off, it is soon replaced by gas having 

 the like high partial pressure of oxygen. Hence we are led to 

 conclude that oxygen makes its appearance in the swim-bladder 

 by a kind of secretion. And other facts might be brought for- 

 ward, strong enough at least to support the doubt, whether the 

 purely physical explanation given above of the entrance of oxy- 

 gen into the blood, adequate as it at first sight seems, is really 

 the true one. 



288. The Exit of Carbonic Acid. In a similar manner 

 analogous experiments appear to support the view that the 

 escape of carbonic acid from the blood into the pulmonary alve- 

 olus is the result of ordinary diffusion ; observations seem to 

 shew that the difference obtained between the pressure of the 

 carbonic acid in the venous blood and the partial pressure of 

 carbonic acid in the air of the pulmonary alveolus (which is of 

 course greater than that of the expired air) is sufficient to 

 account for the loss of carbonic acid, whereby arterial blood is 

 distinguished from venous blood. But in respect to this as in 

 respect to the entrance of oxygen, doubts have been raised, and 

 it has been urged that the escape of carbonic acid into the pul- 

 monary alveoli is carried out by some action of the walls of the 

 alveoli comparable to the act of secretion. 



