622 INCOME AND OUTPUT. [Book ii. 



Comparison of Income and Output of Material. 



§ 415. Method. We have now to inquire how the elements 

 of food are distributed in the excreta, in order that, from the 

 manner of the distribution, we may infer the nature of the 

 intermediate stages which take place within the body. By 

 comparing the ingesta with the excreta, we shall learn what 

 elements have been retained in the body, and what elements 

 appear in the excreta which were not present in the food ; from 

 these we may infer the changes which the body has undergone 

 through the influence of the food. 



In the first place, the real income must be distinguished 

 from the apparent one by the subtraction of the faeces. We 

 have seen that by far the greater part of the faeces is undigested 

 matter, i.e. food which, though placed in the alimentary canal, 

 has not really entered into the body. The share in the fasces 

 taken up by matter which has been excreted from the blood 

 into the alimentary canal, is so small that it may be neglected ; 

 certainly with regard to nitrogen, the whole quantity of this 

 element, which is present in the faeces, may be regarded as 

 indicating simply undigested nitrogenous matter. 



The income, thus corrected, will consist of so much nitrogen, 

 carbon, hydrogen, oxygen, sulphur, phosphorus, saline matters, 

 and water, contained in the proteids, fats, carbohydrates, salts, 

 and water of the food, together with the oxygen absorbed by 

 the lungs, skin, and alimentary canal. The output may be 

 regarded as consisting of (1) the respiratory products of the 

 lungs, skin, and alimentary canal, consisting chiefly of carbonic 

 acid and water, with small quantities of hydrogen and car- 

 buretted hydrogen, these two latter coming exclusively from 

 the alimentary canal ; (2) of perspiration, consisting chiefly of 

 water and salts, for the dubious excretion (see § 350) of urea by 

 the skin may be neglected, and the other organic constituents 

 of sweat amount to very little ; and (3) of the urine, which is 

 assumed to contain all the nitrogen really excreted by the body, 

 besides a large quantity of saline matters and of water. Where 

 great accuracy is required the total nitrogen of the urine ought 

 to be determined ; it is maintained, however, that no errors of 

 serious importance arise when the urea alone, as determined 

 by Liebig's method (which was largely used in the researches 

 forming the basis of the present discussion), is taken as the 

 measure of the total quantity of nitrogen in the urine, since, in 

 this method, other nitrogenous bodies besides urea are precipi- 

 tated, and so contribute to the quantitative result. It has been 

 and indeed still is debated whether the body may not suffer 

 loss of nitrogen by other channels than by the urine and faeces, 

 whether nitrogen may not leave the body by the skin or indeed 

 in a gaseous state by the lungs. The balance of the conflicting 



