PHYSIOLOGY OF MOUTH-BREATHING APPLIANCES. 181 



That it is not wholly explained by this is shown by the fact that the 

 average increase in the rate of walking with the short preliminary 

 breathing period is only about 1 per cent as compared with the actual 

 increment of 2 per cent in the oxygen consumption. The evidence is 

 therefore to the effect that with but 15 seconds of preliminary mouth- 

 piece breathing, a slightly greater oxygen consumption is required 

 during the experimental period than with 15 minutes of preliminary 

 breathing through the mouthpiece. 



The respiratory quotient on the four days is invariably lower with 

 the short period of preliminary breathing. At times the difference is 

 very considerable, even as large as 0.06. It is therefore clear that the 

 mouthpiece breathing is not ideally adapted for an analysis of the 

 character of the combustion when heavy work is being performed. 

 A large number of experiments have been made in the Nutrition Labo- 

 ratory on the comparison of various types of respiration apparatus, 

 using mouthpieces, nosepieces, and masks, and these show that for 

 periods of rest no appreciable difference exists between the various 

 types employed. 1 The true respiratory quotient obtained in these 

 walking experiments is difficult to valuate. A priori, one could take 

 the ground that the longer the mouthpiece was inserted in the mouth, 

 the more normal the respiration would be. But in any event the 

 percentage error is small, probably not over 2 per cent, and the measure- 

 ments of the metabolism under conditions of great physical activity, 

 such as obtained in many of the experiments reported in this book, 

 can hardly be much, if any, inside of this limit of accuracy. The 

 natural conclusion is, therefore, that although practically all of the 

 experiments were made with a short period of preliminary mouthpiece 

 breathing, rather than a 15-minute period of preliminary breathing, 

 and these tests indicate that the metabolism is thereby slightly increased 

 if measured by the oxygen consumption, yet it does not seem advisable 

 to attempt a correction of the results for the small differences shown in 

 this series of tests. 



As previously stated, in actual experiments the mouthpiece is usually 

 inserted about 2 minutes before the observations of the metabolism 

 begin. This preliminary period is measurably greater than the short 

 period in the series of comparison tests, but much shorter than the 

 15-minute periods. Doubtless the error due to the insertion of the 

 mouthpiece is not distributed in a straight line, and it is more than 

 reasonable to suppose that at the end of 2 minutes the oxygen con- 

 sumption is more nearly in accord with that obtained with the longer 

 period of preliminary respiration than with that with the very short 

 period. 



1 Carpenter, Carnegie Inst. Wash. Pub. No. 216, 1915. Also, Hendry, Carpenter, and Emmes, 

 Boston Med. and Surg. Journ., 1919, 181, pp. 285, 334, and 368. 



