622 LECTURE XXVII. 



four hours or less. The metabolism of the food eaten the previous day 

 will affect the results; and, moreover, the metabolism of the food taken 

 during the day of the experiment, will not be complete during such a 

 shorj time. Atwater * showed how much more valuable the results were 

 when the experiment was continued for some considerable time. A 

 great many contradictions and differences in the researches concerning 

 metabolism, which are to be found in the literature, may be traced to the 

 fact that this requirement has not been satisfied. 2 



As a foundation for a metabolic balance, an accurate knowledge of the 

 composition of the income and of the outgo is essential. As regards the 

 income, the foods are to be considered in two directions. We can, on 

 the one hand, evaluate them according to their chemical composition, 

 and, on the other, according to the energy which they contain. We obtain 

 the former values by means of chemical analysis. We have the organic 

 and inorganic constituents to estimate. In the former the carbon, hydrogen, 

 and nitrogen are determined. By multiplying the nitrogen found by 6.25, 

 the amount of albumin is obtained. Of course this method of estimat- 

 ing the amount of albumin is not an exact one. It is perfectly possible that 

 the food may contain nitrogen in some other form than as albumin. In 

 general, however, the amount of such nitrogenous matter is inconsiderable. 

 The fat content is obtained by extracting the food with ether, in which 

 connection it is to be remembered that a part of the fat will go into solu- 

 tion only after the product examined has been " opened up " 3 by digestion 

 with pepsin-hydrochloric acid, or with two per cent hydrochloric acid. 

 Then, when the ash is known, the amount of carbohydrate may be 

 determined by difference. By drying an aliquot part of the weighed 

 mixture, and weighing the dry residue, the amount of water in the food is 

 determined. Oxygen is naturally also to be considered among the sub- 

 stances taken into the system. Unfortunately, up to the present it has 

 not been found possible to measure accurately the amount of this gas 

 that is utilized. This thwarts the exact answering of many questions 

 in the field of metabolism. The potential energy of the food may be 

 ascertained by the heat of combustion. In exact experiments we must 

 naturally also take into consideration the temperature of the food and 

 drink as it is taken into the system. 



1 Ergeb. Physiol. (Asher and Spiro) Jg. 3, 497 (1904). 



2 Studies of metabolism under pathological conditions are important. Clinical 

 investigation and experimental pathology are closely connected with the progress of 

 the knowledge of metabolism under normal conditions. By such means many new 

 questions have arisen, and certain disturbances have given us insight into this or that 

 process. It would be beyond the scope of these lectures to attempt to mention the 

 numerous discoveries which have been made in this way. We can give only the outline 

 here and the more important results. The physiology of metabolism has become such 

 an important branch of science that it can be studied only upon a broad basis. 



3 Cf. Lecture XIV, p. 325. 



