838 SPECIAL PHYSIOLOGY. 



action of false gaseous diffusion and chemical affinity, the escape of car- 

 bonic acid gas from the blood, is perhaps dependent on diffusion only. 

 The accumulation of this gas in the venous blood, owing to chemical 

 processes to be presently mentioned, produces a greater tension in the 

 carbonic acid in the blood, than in that present in the air of the air- 

 cells ; for it is proportionally much more abundant in the former than 

 in the latter. Hence, an outward diffusion of the carbonic acid dis- 

 solved in the venous blood, through the moist walls of the pulmonary 

 capillaries and air-cells, and its escape into the residual air, at the sur- 

 face of the lining membrane of the cells. The carbonic acid thus dis- 

 solved in the blood is chiefly contained in a state of solution in the 

 liquor sanguinis, the red corpuscles having no special affinity for it. 

 The absorption and evolution of nitrogen in the respiratory process, 

 are accomplished also by moist diffusion. 



Two points yet remain for consideration, viz., In what part of the 

 circulation, and at the expense of what constituents of the blood and 

 tissues, does the oxygen absorbed in respiration, become united with 

 carbon, to produce the carbonic acid given off? The answer to these 

 questions constitutes an important part of the theory of respiration. 

 Previously to the discovery of oxygen, nitrogen, and carbonic acid, all 

 explanations of the respiratory process were necessarily vague. The 

 earlier physiologists believed that the air the source, as they deemed 

 it, of the animal spirits found its way through the lungs, and obtained 

 an entrance, as such, into the so-called arteries. 



Oxygen, or phlogiston, was discovered by Priestley and Scheele, in 

 1774. Black had already described what he called fixed air, and 

 Rutherford had determined the existence, in air respired by an animal, 

 of a peculiar gas incapable of supporting further respiration, or com- 

 bustion. Lavoisier named the gas first discovered by Priestley and 

 Scheele, oxygen; by him, also, the gas described by Rutherford, now 

 more commonly known by the name of nitrogen, given to it by Chaptal, 

 from its being contained in nitre, was named azote (a, not, and zoe, 

 life), from its inability to support life, in respiration ; lastly, Lavoisier 

 demonstrated that the fixed air of Black, now shown to be produced 

 alike by the action of acids on limestone, by combustion, fermentation, 

 and respiration, contains the element carbon. These great discoveries 

 were indeed the commencement of the modern science of Chemistry, 

 and the foundation of all true chemical theory; and Lavoisier himself, 

 in combining and adding to the knowledge of his predecessors, at once 

 applied the results to explain the respiratory process of animal life, 

 and offered to science the first theory of respiration. 



Lavoisier saw that the oxygen absorbed in respiration, united, in 

 some way and somewhere, with carbon, to produce the carbonic acid 

 evolved; he regarded the process as a sort of combustion, and supposed 

 that the combination took place in the lungs, i. #., in the pulmonary 

 capillaries, in which the obvious change from venous to arterial blood 

 occurs ; the oxygen was thought to be there immediately transformed 

 into carbonic acid, and then as immediately given off. But later in- 

 quiries have shown that this view must undergo modification. It was 

 not known to Lavoisier, that both venous and arterial blood contain 





