4 PHYSIOLOGY CHAP. 



(c) There may be a deficit in the balance when the output of 

 material exceeds the intake, so that the weight of the body 

 diminishes more or less slowly from day to day. This occurs 

 during the cataplastic or involutionary period of old age, total or 

 partial inanition, periods of insufficient nutrition, and during 

 acute or chronic wasting diseases. 



The methodical testing of the total weight of the body at 

 regular intervals will therefore suffice to give a rough idea of the 

 total exchange of material, to tell us whether the total output is 

 much the same as the intake, or whether there is a considerable 

 surplus or gradual deficit in the balance. The comparison of the 

 separate elements of the material taken in and discharged is, 

 however, of much greater interest. It enables us up to a certain 

 point to ascertain what substances are formed or consumed in the 

 organism. Thus, for instance, from a separate balance of the 

 intake and output of nitrogen and carbon we can reckon approxi- 

 mately, as we shall see later, the quantity of protein and fat 

 formed and consumed from day to day in subjects under definite 

 conditions of environment and diet. 



We must, however, point out that the majority of the results 

 obtained in these researches on metabolism by numerous experi- 

 menters have a merely approximate value and must be accepted 

 with caution, because the methods employed cannot give absolutely 

 accurate numerical data ; and small mistakes in data, grown more 

 serious during calculation, may lead to altogether wrong con- 

 clusions. The recent perfecting of methods of research and the 

 estimation of their inherent inaccuracies have, however, effected 

 a great advance in this vast field of physiological investigation. 

 These researches enable us to determine the conditions favourable 

 to the disintegration or reintegration of the tissues, especially the 

 development or atrophy of the muscles which form but little less 

 than half the mass of the body, and the deposition or consumption 

 of fat ; to estimate the nutritive value and, generally, the different 

 uses of the various groups of food-stuffs ; to establish the basis for 

 the theory of nutrition, i.e. to determine the average alimentary 

 regimen of man ; and, finally, to ascertain the internal regulating 

 forces and, as far as possible, the laws of chemical changes in the 

 animal economy. 



II. The animal's daily intake of material is represented by the 

 food taken and the oxygen absorbed by pulmonary and cutaneous 

 respiration ; the output, on the other hand, by the products of 

 consumption discharged by the kidneys, intestines, lungs, and skin. 

 An exact determination of the intake would require a complete 

 chemical analysis of the food, in order to ascertain the quality and 

 quantity of protein, fat, and carbohydrate contained in it an 

 analysis which would demand too much time and labour, and has 

 therefore never been made by any experimenter. In order to 



