232 COMPARISONS OF RESPIRATORY EXCHANGE. 



the beginning and end of the experimental period, a practice which we 

 have found very disadvantageous and liable to result in disturbing 

 the normal respiration; it is desirable to conduct the experiment in 

 such a manner that the subject has no knowledge as to the beginning 

 and end of the periods. Furthermore, there is a question as to whether 

 it is possible for the subject to expire voluntarily and forcibly to exactly 

 the same point each time. Such a procedure would require con- 

 siderable practice and the position in which the subject usually lies, 

 i. e., on his back, is not conducive to a perfect forced expiration. It 

 is also necessary for the operator to turn the valve at exactly the 

 moment when the subject has ended this forced expiration; this may 

 make it necessary for him to hold this position until the valve is turned. 

 In Holly's modification there is no control on the accuracy of this 

 valve movement; in the spirometer type of the Benedict respiration 

 apparatus, an admirable control has been established on the turning of 

 the valve at the exact end of the expiration. 



This question of the volume of air in the lungs of the subject at the 

 beginning and end of the experimental period is of the most vital 

 importance in determining the oxygen consumption by the Benedict 

 apparatus. After all other sources of error are eliminated, it remains 

 the most important assumption bearing upon the fundamental prin- 

 ciple of the determination of the oxygen consumption. In the earlier 

 development of the apparatus this was apparently not a serious matter, 

 as most of the subjects were more or less trained to breathing on res- 

 piration apparatus and accordingly breathed regularly and quietly, 

 without an apparent variation in the volume of the lungs. When the 

 apparatus was used with subjects who were unaccustomed to it this 

 factor was somewhat more in evidence and, in many instances, it 

 was apparent that the subjects were not breathing normally and 

 regularly and that the volume of air in the lungs must be continually 

 changing. In a study of the influence of a no-carbohydrate diet, it 

 was found impossible to use a certain subject, owing to the fact that in 

 several tests made with him, he apparently constantly increased the 

 volume of air in the system in breathing instead of reducing it. The 

 fact that we were not obtaining trustworthy results agitated him 

 and this caused even greater disturbances. Attempts were made at 

 different times of the day to secure better results, but without marked 

 success. There have also been other cases when it was very difficult 

 to obtain uniform results. A comparison of the probability curves 

 for the respiratory quotients obtained with the Benedict, Zuntz, and 

 Tissot apparatus show that both with the Zuntz and with the Tissot 

 apparatus the respiratory quotients are more uniform than with the 

 Benedict apparatus. As the experiments in which these apparatus 

 were compared were carried out under the same conditions, the lesser 

 degree of uniformity with the Benedict apparatus is probably due to 



