THE STATISTICS OF NUTRITION. 485 



Comparison of Income and Output of Material. 



435. Method. We have now to inquire how the elements of food are 

 distributed in the excreta, in order that, from the manner of the distribu- 

 tion, 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 ap- 

 parent one by the subtraction of the feces. We have seen that by far the 

 greater part of the feces is undigested matter, i. e. t food which, though 

 placed in the alimentary canal, has not really entered into the body. The 

 share in the feces taken up by matter which has been excreted* from the 

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

 tainly, with regard to nitrogen, the whole quantity of this element, which 

 is present in the feces, may be regarded as indicating simply undigested 

 nitrogenous matter. 



The income, thus corrected, will consist of so much nitrogen, carbon, hy- 

 drogen, 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 

 Doming exclusively from the alimentary canal ; (2) of perspiration, consist- 

 ing chiefly of water and salts, for the dubious excretion (see 366) 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 precipitated, 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 feces, whether nitrogen' 

 may not leave the body by the skin, or, indeed, in a gaseous state, by the 

 lungs. The balance of the conflicting evidence seems, however, in favor 

 of the view that no such loss takes place. It would appear that though 

 nitrogen, the pivot, so to speak, of the chemical changes of living beings, 

 forms so large a portion of the atmosphere, and, moreover, is physically 

 diffused through the bodies of both plants and animals, free nitrogen is of 

 no chemical use to either of them. It enters into and remains in their 

 bodies as an inert substance, and the nitrogen which leaves a plant or ani- 

 mal, in a gaseous state, is simply a part of the same inert supply, and does 

 not come from the breaking up of the nitrogenous substances of the body 

 or of the food. 



Of these elements of the income and output, the nitrogen, the carbon, 

 and the free oxygen of respiration are by far the most important. Since 

 water is of use to the body for merely mechanical purposes, and not solely 

 as food in the strict sense of the word, the hydrogen element becomes a 



