xi EESPIKATOEY EXCHANGES 387 



serum, on the contrary, with the simple vacuum, it is only possible 

 to extract one part, and the addition of a weak acid is required to 

 extract the residue, which is more stably combined, and is present 

 in the blood to the amount of 5-9 vols. per cent (Pfiliger). The 

 fact that this portion also is turned out, in vacuo, in the presence 

 of corpuscles, without adding acid, suggests that the corpuscles 

 contain substances that function as acids, and that these are 

 diffused into the plasma during the action of the vacuum, or that 

 th$ sodic carbonate of the plasma penetrates to the corpuscles. 

 Among the acids of the corpuscles the first place must be given to 

 phosphoric anhydride, which they contain in larger quantities than 

 the plasma; besides which the oxyhaemoglobin functions as an 

 acid, as was demonstrated by Preyer, since it is capable of liberat- 

 ing carbonic acid from its sodium combinations in vacuo. 



In regard to nitrogen and argon, we must 'confine ourselves to 

 saying that these gases are found in the blood in amounts differing 

 little from those in which they are absorbed and dissolved by 

 watery fluids in the presence of atmospheric air. According to 

 Kegnard and Schloesing, about 0'04 vol. per cent of the 2 vols. per 

 cent of indifferent gases extracted from the blood are argon. The 

 opinion held by some that a small amount of free nitrogen is 

 developed during the oxidative processes of the nitrogenous 

 substances of the tissues, and is subsequently poured into the 

 blood, has not at present been confirmed by any incontrovertible 

 evidence. Regnault and Keiset found a slight increase of nitrogen 

 in expired as compared with inspired air. So, too, the nitrogen 

 extractible from venous blood is always somewhat greater than 

 that which can be extracted from arterial blood. Pettenkofer 

 and Voit gave an adequate explanation of these facts, on the 

 assumption that they depend on the swallowing of air with 

 the food, and on the absorption of the nitrogen contained in the 

 gases of the intestines. 



VIII. Since both oxygen and carbonic acid are thus found in 

 the blood in the form of readily dissociable combinations (in 

 relation to variations of partial pressure), it is natural to conclude 

 that the gas exchanges which take place incessantly between the 

 blood circulating through the capillaries of the lungs and the air 

 contained in the pulmonary alveoli (external respiration) are 

 accomplished by a simple physical process of diffusion, regulated 

 by Dalton's law. They depend, i.e., on the difference of the partial 

 pressures of the said gases as contained in the fluid and gaseous 

 media, separated by permeable septa or membranes, formed by the 

 walls of the capillaries and the epithelial cells that line the alveoli. 



The scientific demonstration of this theory involved a series of 

 researches, directed to the separate determination of the partial 

 pressures of the two gases in venous and in arterial blood, in order 

 to compare them with those of inspired and expired air. 



