THE CONTROL OF THE RESPIRATION 357 



the method by which the tensions of these gases in alveolar air can be 

 determined. The simplest and until recently the most accurate method 

 is that of Haldane and Priestley. 19 This consists in having an individual, 

 with his nostrils clamped, breathe quietly through a piece of hose pipe 

 about a meter long, which has at the mouth end a short side-tube lead- 

 ing to an evacuated gas-sampling bulb of about 50 c.c. capacity.* (Fig. 

 124). After the subject has become accustomed to breathing through the 

 tube, he is asked to make a forced expiration and at the end of it to close 

 the mouthpiece with his tongue. . At this moment the operator opens the 

 tap of the sampling tube, allowing the air from the tubing through 

 which the individual has made the forced expiration to rush in and fill 

 it. This sample represents the air from the alveoli (see page 319), and 

 is analyzed for percentages of C0 2 and 2 . Since each normal inspira- 

 tion dilutes the alveolar air somewhat, it is necessary, for constant re- 



Fig. 124. Apparatus for collection of a sample of alveolar air by Haldane's method. It is 

 better to use a mouthpiece than a mask. 



suits, to make two analyses of alveolar air from each subject, one taken 

 at the end of a normal inspiration and the other at the end of normal 

 expiration. The average of the two results is taken as the composition 

 of the alveolar air. 



On account of the difficulty in securing intelligent cooperation in the 

 application of this method, particularly with children, other methods have 

 been devised. One of the simplest is that of Fridericia, which is a modifica- 

 tion of the Haldane-Priestley method, the apparatus for which is shown 

 in the figure (Fig. 125), and the manipulation of which is outlined in 

 the legend. Another is to take a mixed sample of the very last portion 

 of several normal expirations. On account of the extended use which is 

 being made of measurements of alveolar air composition, both in lab- 



*In place of the gas-sampling tube it is much more convenient and equally accurate to employ one 

 of the modern ground glass piston syringes (Luer). The piston should, of course, be well smeared 

 with a good mineral grease. 



