458 RESPIRATION. 



received by physiologists, as representing the relative propor- 

 tions of the gases in the two kinds of blood, though Magnus 

 states in his paper that he does not think he succeeded in 

 extracting all the gas the blood contained. 1 It is a question 

 of the last importance, as bearing upon our comprehen- 

 sion of the essential processes of respiration, to be able to 

 determine the relative proportion of oxygen and carbonic 

 acid in the arterial and venous blood. Until very recently, 

 our ideas on this subject have had for their sole experimental 

 basis the observations of Magnus, and in discussing the accu- 

 racy of the modes of analysis of the blood for gases we need 

 take no account of any experiments anterior to his. 



Analysis of the Blood for Gases. There are 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 paper shows another 

 element of inaccuracy which is even more important. The 

 relative quantities of oxygen and carbonic acid 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 impossible 

 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 displacement is employed, 2 and the 

 bubbling of the gas when extracted by the air-pump, this 



1 The original article of Magnus is published in the Annalen der Physik und 

 Chemie of Poggendorff, April, 1837, and is translated into French in the Ann. de 

 Chimie et de Phys. of the same year. 



2 When a gas, such as hydrogen, which is not contained in the blood, is thor- 

 oughly mixed with it by agitation in a closed vessel, it will penetrate the liquid, 

 and displace, or drive off, all the free gas which is held in solution. This is called 

 the method of analysis by displacement. 



