26 RESPIRATORY EXCHANGE OF ANIMALS AND MAN 



to maintain a low and practically constant percentage of carbon di- 

 oxide in the air. This involves a gas volume which is large com- 

 pared with the animal experimented on (not less than 100 times its 

 volume), and when the gas volume exceeds a couple of 100 c.c. it will 

 no longer be sure to possess a uniform temperature and composition 

 throughout. 



In the micro-respiration apparatus the oxygen absorbed is not 

 replaced, but on account of the comparatively large volume of en- 

 closed air the changes in composition are always very slight. 



In all larger closed-space respiration apparatus we have a sepa- 

 rate absorbing system for the carbon dioxide connected by tubing 

 with the animal chamber. This involves further a ventilating arrange - 



FIG. 5. Krogh's respiration apparatus for cold-blooded animals. From " Zeitschr. 

 f. Physik. Chem. Biologie ". 



ment to bring the air from the animal chamber to the absorber and 

 back again. 



The simplest form which is at the same time very convenient and ac- 

 curate is Regnault-Reiset's apparatus for small animals [1849], especi- 

 ally when combined with a compensating vessel (Krogh [1914]), as 

 shown in fig. 5. In this instrument the CO 2 absorbing and ventilating 

 arrangements are combined into one (2). The oxygen absorbed is 

 measured in the burette (6) when the manometer (5) has been brought 

 back to its zero point just as in Winterstein's micro-respiration ap- 

 paratus. When the mercury has reached the top of the burette, 

 oxygen is added through (7) and the mercury brought back to the 

 o mark on the burette. The use of a water current as a source of 



