BETWEEN THE BLOOD AND ALVEOLAR AIR 543 



the causes of the gaseous exchange between the blood and the 

 alveolar air, for the experimental results obtained by different 

 observers are at variance. In a case of such difficulty and im- 

 portance it is best to state the arguments on each side, sum up 

 the evidence, and indicate as far as possible what verdict should 

 be given upon the balance of evidence. The verdict may or may 

 not be true, for the evidence appeals differently to each observer, 

 and it is difficult to separate from the consideration of this subject 

 the personal attitude of each man towards the Materialistic and 

 Vitalistic Schools of Physiology. It may be that a verdict of 

 " not proven " will be more in agreement with the nature of the 

 evidence. 



The two cases, stated briefly, are as follows : the gaseous 

 exchange between the blood and the alveolar air is due to the 

 relative partial pressures of the gases in the blood and alveolar 

 air, and can be explained according to physical and chemical laws ; 

 the gaseous exchange takes place in opposition to the known 

 physical and chemical laws, is of a special nature, a vital process 

 akin to the secretion and excretion of glands. 



The solution of the question depends upon a comparison of 

 the partial pressures of the gases in the arterial and venous blood 

 with those of the corresponding gases in the alveolar air. It has 

 already been shown how samples of alveolar air may be obtained 

 under ordinary atmospheric pressure, and yield on analysis the 

 partial pressures of their constituent gases : the methods used for 

 the determination of the pressures of the gases in the blood have 

 not yet been described. For the determination of these pressures 

 Pfliiger introduced an instrument known as the aerotonometer, the 

 principle of which is as follows : blood in contact with a mixture 

 of oxygen, carbon dioxide, and nitrogen absorbs or gives up gas, 

 according as the pressures of its respective gases are less or greater 

 than those of the corresponding gases in the mixture. If sufficient 

 time be allowed, this interchange will result in equilibrium ; the 

 partial pressures of the gases in and outside the liquid will be the 

 same, and thus from an analysis of the mixture at known pressure 

 it is possible to determine the partial pressures of the gases in 

 the blood. 



In the aerotonometer the blood passes in a very shallow stream 

 through a glass tube which contains a mixture of gases of known 

 quantities and pressure, and is surrounded by a water-jacket with 



