Introduction. 3 



and actual conditions. This is particularly the case when only the gross income, 

 i. e., the weight of food and drink is known, and the changes in body-weight 

 are used as an index of gain or loss of body-material. The large proportion 

 of water in the body may permit of a considerable draft upon it or an addition 

 thereto may be made, and while the body may have gained 1 kilogram in 

 weight this gain is not necessarily due to increase of organic material either 

 protein, fat, or carbohydrate but it may be wholly water, or part water and 

 part organic material, or it may even be an actual loss in organic material, 

 compensated by an extra gain in water. 



The character of the diet influences to a great extent the amount of the 

 water in the bod}', as, for example, a diet consisting in large part of carbo- 

 hydrates results in a considerable storage of preformed water in the body 

 water that is rapidly excreted upon substituting a diet in which fat predom- 

 inates. 1 Changes in body-weight can accordingly be considered as a true index 

 of body condition only when the records cover considerable periods of time. 



Studies of the Respiratory Exchange. 



While the data for the normal pulse-rate, respiration-rate, and body-tem- 

 perature are, in a large number of instances, obtained under conditions that 

 would seriously affect the results, they are infinitely more firmly established 

 as physiological factors than is, as yet, our knowledge of the oxidation proc- 

 esses and energy transformations. The observations regarding the pulse-rate, 

 temperature, respiration-rate, muscular activity, changes in body-weight, and 

 even analyses of urine, feces, and food, do not require any especially complex 

 or costly apparatus. If, however, to this somewhat meager balance of income 

 and outgo it is desired to add information regarding the respiratory gases 

 and particularly the heat transformations, none of the ordinary methods of 

 clinical study suffice, and we must resort to elaborate and more or less costly 

 apparatus requiring special technique for its successful use. In order to collect 

 the respiratory gases, the patient must either breathe through a special appli- 

 ance, such as nosepiece or mouthpiece, or else he must be wholly or in part 

 incased in a large chamber. If energy transformations are to be studied, 

 calorimetric apparatus of considerable complexity is necessary. 



Early contributions. It was very early noticed that the excretion of carbon 

 dioxide fluctuated rapidly with variations in body activity and variations in 

 food. Carbon dioxide as a product of combustion appeared to be an ideal 

 quantitative index of the combustion inside the body. Physiological chemists 

 began early to make measurements of this gaseous product, and, owing to the 

 fact that the determination of carbon dioxide is relatively easy and accurate, 

 the results have even to-day a practical significance. 



1 For a striking illustration of the influence of the character of the diet upon the 

 body-weight, see Benedict and Milner, U. S. Dept. Agr., Office Exp. Stas. Bui. 175, 

 p. 225, 1907. 



