INTRODUCTION. 



THE respiratory exchange of an animal in the widest sense of the 

 term means the exchange of gaseous substances taking place between 

 the organism and the surrounding atmosphere, and it is defined not by 

 any physiological difference between the part played within the body 

 by these substances and others but solely by practical considerations 

 of convenience. The investigation of the gaseous exchange requires 

 special methods and a technique which is rather different from that 

 employed in other branches of biochemical work. In most cases 

 quantitative determinations of gaseous exchange can be made only 

 by means of more or less complicated respiration apparatus, the 

 manipulation of which is supposed to require a great deal of special 

 training. 



The study of the gas exchange of organisms dates back very far, but 

 the difficulties in dealing with gases delayed progress for a very long 

 time and are responsible to a certain extent for the present unsatis- 

 factory state of our knowledge regarding some of the fundamental 

 problems encountered. 



It was first shown by John Mayow [1668] that a gas spiritus 

 nitro-aereus = oxygen is constantly absorbed by animal organisms. 

 Nearly a hundred years later [1757] Black discovered that "fixed air," 

 what we now call carbon dioxide, is produced and eliminated during 

 expiration ; but Lavoisier [1777] was the first to understand the signi- 

 ficance of the respiratory exchange as the result of a process of 

 combustion going on within the body, and by which the carbon and 

 hydrogen of the animal tissues are combined with oxygen to form 

 carbon dioxide and water. Lavoisier was the first also to make 

 quantitative measurements of the oxygen absorption and CO 2 eli- 

 mination, and so to determine the respiratory exchange in the sense 

 in which this term is usually employed. 



Lavoisier and Seguin [1817] stated expressly that the nitrogen ot 

 the atmosphere does not take part in the respiratory processes, and for a 

 long time these were taken to consist exclusively in the exchange of 

 oxygen and carbon dioxide. By the famous researches of Regnault and 



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