CHANGES OF THE BLOOD IN RESPIRATION. 151 



essential changes which this fluid undergoes in respiration are to be looked 

 for in connection with the proportions of oxygen and carbon dioxide before 

 and after it has passed through the lungs. 



The change of color in the blood from dark-blue to red, in its passage 

 through the lungs, was recognized by Lower, Goodwyn and others, as due 

 to the action of the air, long before the discovery of oxygen. Since the 

 discovery of oxygen, it has been ascertained that this is the only constituent 

 of the air which is capable of arterializing the blood. Priestley showed 

 that venous blood is not changed in color by nitrogen, hydrogen or car- 

 bon dioxide ; while all these gases, by displacing oxygen, will change the 

 arterial blood from red to black. Carbon monoxide, although it is not a 

 respirable gas and does not properly arterialize the blood, changes it from 

 black to red. 



The elements of the blood which absorb the greater part of the oxygen 

 are the red corpuscles. While the plasma will absorb, perhaps, twice as much 

 gas as pure water, it has been shown that the volume of oxygen fixed by the 

 corpuscles is about twenty-five times that which is dissolved in the plasma 

 (Fernet, Lothar Meyer). 



Comparison of the Gases in Venous and Arterial Blood. The demon- 

 stration of the fact that oxygen and carbon dioxide exist in the blood, with a 

 knowledge of the relative proportion of these gases in the blood before and 

 after its passage through the lungs, are points hardly second in importance to 

 the relative composition of the air before and after respiration. The idea enun- 

 ciated by Mayow, about two hundred years ago, that " there is something in 

 the air, absolutely necessary to life, which is conveyed into the blood," except 

 that the vivifying principle was not named or its other properties described, 

 expresses what is now regarded as one of the great objects of respiration. 

 This is even more strictly in accordance with facts than the idea of Lavoisier, 

 who supposed that all the chemical processes of respiration took place in the 

 lungs. Mayow also described the evolution of gas from blood placed in a 

 vacuum. Many observers have since succeeded in extracting gases from 

 the blood by various processes ; but notwithstanding this, before the experi- 

 ments of Magnus, in 1837, many denied the existence of free gases in the 

 blood. 



Analysis of the Blood for Gases. There were certain grave sources of 

 error in the method employed by Magnus, which render his observations of 

 little value, except as demonstrating that oxygen, carbon dioxide and nitro- 

 gen may be extracted by the air-pump from both arterial and venous blood. 

 The only source of error in the results which he fully recognized lay in the 

 difficulty in extracting the entire quantity of gas ; but a careful study of his 

 essay shows another element of inaccuracy which is even more important. 

 The relative quantities of oxygen and carbon dioxide in any single specimen 

 of blood present great variations, dependent upon the length of time that the 

 blood has been allowed to stand before the estimate of the gases is made. As 

 it is difficult to make this estimate immediately after the blood is drawn, on 

 account of the froth produced by agitation with a gas when the method by 



