156 RESPIRATION. 



in the pulmonary vein. Adapting a bellows to the trachea, we imitate the process of 

 respiration ; and, if defibrinated venous blood be carefully injected through the lungs, it 

 will be returned by the pulmonary vein, presenting the bright-red color of arterial blood. 

 When the artificial respiration is interrupted, the blood passes through the lungs without 

 change. 1 In exposing the thoracic organs and keeping up artificial respiration, repeating 

 the celebrated experiment of Robert Hook, made before the Royal Society, in 1664, we 

 can see, through the thin walls of the auricles, the red color of the blood on the left side 

 contrasting with the dark venous blood on the right. 



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 carbonic acid ; while all these gases, 

 by displacing oxygen, will change the arterial blood from red to black. 2 



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 by Magnus and by Gay-Lussac that the corpuscles will absorb from ten to 

 thirteen times as much. By some the proportion is put much higher. According to the 

 late researches of Fernet, which have been confirmed by Lothar Meyer, the volume of 

 oxygen fixed by the corpuscles is about twenty-five times that which is dissolved in the 

 plasma. 



Comparison of the Gases in Venous and Arterial Blood. The demonstration of the 

 fact that free oxygen and carbonic acid exist in the blood, with a knowledge of the rela- 

 tive proportion of these gases in the blood before and after its passage through the lungs, 

 is a point hardly second in importance to the relative composition of the air before and 

 after respiration. The idea enunciated 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 is not named or its other properties described, 

 expresses what we now consider 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 de- 

 scribed the evolution of gas from blood placed in a vacuum. Many observers have since 

 succeeded in extracting gases from the blood by various processes. Sir Humphry Davy 

 induced the evolution of carbonic acid by raising arterial blood to the temperature of 200 

 Fahr., and venous blood to a temperature of 112; Stevens and others disengaged gas 

 by displacement with hydrogen, nitrogen, or the ordinary atmosphere; but, notwith- 

 standing this, before the experiments of Magnus, in 1837, many denied the existence in 

 the blood of any free gas whatsoever. 



Analysis of the Blood for Oases. 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, carbonic acid, and nitrogen 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 in 

 solution ; but a careful study of his essay shows another element of inaccuracy which is 

 even more important. The relative quantities of oxygen and carbonic acid in any single 



1 This demonstration is very striking, especially if we use a syringe with a double nozzle, one point secured in the 

 pulmonary artery, and the other simply carrying the blood by a rubber tube into a glass vessel. Keceiving the blood 

 which passes through the lungs and that which simply passes through the tube, into two tall glass vessels, the one 

 is of a bright red, and the other retains its dark color. In preparing for the experiment it is necessary, immediately 

 after removing the lungs from the animal, to inject them with a little defibrinated blood, so as to remove the coagu- 

 lating blood from the pulmonary capillaries, which would otherwise become obstructed. The injection should be 

 made gently and gradually, to avoid extravasation. Defibrinated ox-blood may be used. The most convenient way 

 to secure the canulse in the vessels is to push them into the pulmonary artery through the right ventricle, and into 

 the pulmonary vein through the left auricle. 



2 Carbonic oxide and nitrous oxide have a strong affinity for the blood-corpuscles and become fixed in them, the 

 former giving the blood a vivid red color. Sugar and many salts will also redden venous blood. These agents, how- 

 ever, do not impart the physiological properties of arterial blood. 



