238 SCIENCE PROGRESS. 



may be taken as about "ooi mm. One cc. of blood spread 

 out in a layer of equal thickness would cover an area of 

 10,000 square centimetres, or 1 1 square feet. Venous blood 

 spread out in such a thin film would almost instantaneously 

 become saturated with air. The change from venous to 

 arterial colour can be seen to occur within a second or two 

 even with far thicker films. It is true that in the case of 

 such films the blood is in immediate contact with the air, 

 while in the lungs there is a thin layer of protoplasm 

 between the blood and the air ; but the difference from a 

 purely physical point of view is more apparent than real, 

 since even in the case of the exposed film albuminous liquid 

 surrounds the red corpuscles which take up nearly all the 

 oxygen from the air. 



There is thus not the slightest difficulty in imagining a 

 physical mechanism by which the interchange of gases 

 between the blood and the air is effected in the lung. The 

 frequency with which formerly accepted physical theories of 

 other physiological processes have proved on experimental 

 investigation to be insufficient has, however, caused the 

 diffusion theory of respiratory exchange in the lungs to be 

 regarded with considerable suspicion ; and I propose to 

 give an outline of the results of the investigations to which 

 these suspicions have given rise. 



It is clear that if the diffusion theory be correct, the 

 partial pressure or tension of the oxygen in solution in the 

 blood leaving the capillaries of the lung alveoli cannot be 

 higher than in the alveolar air : also that the tension of 

 carbonic acid cannot be lower in the blood than in the 

 alveolar air. Thus on the diffusion theory in arterial blood 

 the oxygen tension ought always to be equal to or lower 

 than, and the carbonic acid tension equal to or higher than, 

 that of the alveolar air. 



The tension of carbonic acid in blood flowing from the 

 vessels was first investigated by Pfliiger and three of his 

 pupils 1 (Wolffberg, Strassburg, and Nussbaum), who strongly 

 support the diffusion theory. The apparatus devised by 



1 Pflugers Archiv, vol. iv., p. 465 ; vol. vi., pp. 23 and 65, and vol. vii., 

 p. 296. 



