CHAPTER II. 



THE METHODS EMPLOYED FOR MEASURING THE RESPIRATORY 



EXCHANGE. 



General Principles. 



BEFORE entering upon a review and discussion of the numerous 

 methods and technical appliances used for the purpose of studying the 

 respiratory exchange, it is desirable to emphasize certain principles 

 which are, perhaps, more easily overlooked in gas exchange work 

 than in any other ; they are not peculiar to the study of the gas ex- 

 change, but are common to all scientific measurements. 



It is a general principle that the accuracy of the measurements and 

 the definition of the experimental conditions should correspond to each 

 other. 1 In determinations of the respiratory exchange the conditions 

 are usually very difficult to define. Determinations are usually made 

 on "normal " animals, but a "normal animal" is an extremely vague 

 definition. In a normal animal the respiratory exchange may often 

 vary 100 per cent, and more, and to use methods, the results of which 

 are reliable to within I or 5 per cent., in such a case is obviously waste 

 of time. In most cases, however, the fault lies not so much in the 

 methods being too fine, as in the definition of conditions which might 

 be made more precise. 



It is well known that muscular movements increase the metabolism 

 considerably, and it is obvious that the investigation of the influence of 

 other less potent factors must be made when muscular movements have 

 been excluded. In experiments on animals this principle has very 

 often been disregarded, and one of the most important improvements 

 in technique is undoubtedly the introduction by Benedict and Romans 

 [191 1 ] of the recording cage (fig. 2 and fig. 6), which is put up in the 

 animal chamber of a respiration apparatus, and which will record any 

 shifting of the centre of gravity of the animal under experiment. 

 Periods can then be selected during which the animal keeps quiet. 



J Ostwald- Luther, " Physiko-chemische Messungen," 3 Aufl., Leipzig, 1910. 



