CHAP, v.] NUTRITION. 791 



excretion of urea to which we have already ( 487) referred and 

 to which we shall have to return in speaking of what is known as 

 " luxus-consumption." 



Comparison of Income and Output of Material. 



521. 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 nitro- 

 genous 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 carburetted hydrogen, these 

 two latter coming exclusively from the alimentary canal ; (2) of 

 perspiration, consisting chiefly of water and salts, for the dubious 

 excretion (see 438) 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 



