558 



METABOLISM 



the sample of air. A scale on the side of the instrument gives the vol- 

 ume of the air. 



During an observation the subject sits in a reclining position or lies 

 upon a couch. When the bell of the spirometer is placed at zero, the 

 mouthpiece adjusted in the mouth, and the nose clamped, respiration is 

 started, the expirations being passed through the stopcock, which is 

 so turned as to allow them to pass to the outside air. After a few 

 minutes the stopcock is turned so that the expirations are passed into 



Fig. 180. The Douglas bag method for determining the respiratory exchange. The arrange- 

 ment of mouthpiece, valves, and connecting tubes shown here has been 'found to be more con- 

 venient than that recommended by Douglas. 



the spirometer for a definite length of time. At the end of the period 

 the cock is again turned, and after the barometric pressure, temperature, 

 and volume of the air have been noted, the composition of the air is 

 determined in the Haldane gas analysis apparatus. 



4. THE DOUGLAS BAG. The Douglas bag is made of rubber-lined cloth, 

 and is capable of holding from 50 to 100 liters. It is especially useful 

 for investigations during exercise, since it is fitted with straps so that 

 the bag can be fastened to the shoulders (Fig. 180). It is then connected 

 with the valves, the mouthpiece of which is placed between the lips. 



