RESPIRATION 231 



a slight negative pressure is set up in the main air circuit, when 

 it will pass by preference through M. Such a negative pressure 

 obtains in the main air circuit only at the end of an inspiration, 

 and depends upon the fact that the whole volume of air in the cir- 

 cuit is diminished by the amount of oxygen absorbed at the last 

 breath of the subject, as the carbonic acid expired is removed. The 

 meter records, therefore, the actual oxygen consumption by the 

 individual. Interposed between the meter and the valve M is a 

 small rubber bag L, such as is used in a small sized football. This 

 serves as a reservoir for the oxygen, and enables a free and sudden 

 supply to be drawn into the air circuit. Without this it would be 

 necessary to run the oxygen from the generator at an excessive 

 and wasteful rate, and the slight resistance of the meter might 

 be felt. In practice the oxygen supply is so adjusted that it is just 

 escaping continuously to air through J, so as to insure that the bag 

 L is filled to constant pressure; otherwise the readings of the 

 meter will not accurately represent the oxygen consumption. 



A Haldane gas analysis apparatus N is attached directly to the 

 air pipe leading from the bottle E to the inspiratory valve, so that 

 samples of the inspired air may be withdrawn at intervals during 

 the experiment for analysis. The extremity of a vacuous gas 

 sampling tube O is inserted into the pipe between the expiratory 

 valve and the caustic soda tin, not far from the former, for the 

 purpose of obtaining a sample of alveolar air by Haldane and 

 Priestley's method. By means of the tap P, connected with the 

 laboratory water supply, a large volume of air can be displaced 

 from the bottle E through the pipe R, and used for filling satu- 

 rating vessels, etc. Before each experiment the apparatus is tested 

 for air-tightness by disconnecting the oxygen supply pipe at F 

 and substituting a water manometer for it, and then producing a 

 positive or negative pressure by blowing in air or sucking it out 

 through the mouthpiece. The whole apparatus is readily blown 

 out with fresh air by disconnecting the return air pipe from the in- 

 spiratory valve and blowing through the mouthpiece with a pair 

 of bellows. 



We found that the percentage of oxygen in the air in the ap- 

 paratus falls by about 0.8 per cent during the first five minutes of 

 an experiment, doubtless owing to the rise of temperature caused 

 by the breathing, which will hinder the entrance of oxygen. After 

 this the oxygen percentage shows oscillations, which however do 

 not exceed i per cent. Such oscillations are unavoidable, seeing 

 that the oxygen supply must be influenced in this method by the 



