CHAP, v.] NUTRITION. 827 



In the first place, the real income must be distinguished from 

 the apparent one by the subtraction of the faeces. We have seen 

 that the greater part of the faeces is probably undigested matter, 

 i.e. food which, though placed in the alimentary canal, has not 

 really entered into the body. Though we do not know exactly 

 the share in the faeces taken up by matter which has been excreted 

 from the blood into the alimentary canal, we may, without great 

 error, consider the whole faeces as so much loss of income. 



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. In the earlier observations the urea alone, as 

 determined by Liebig's method, was taken as the measure of the 

 total quantity of nitrogen in the urine ; but, though probably 

 no greater error was thus introduced, the better way is to determine 

 directly the total nitrogen as may readily be done, for instance, by 

 Kjeldahl's method. 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 evidence seems however in favour 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 animal, 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 dubious one ; the sulphur of the 

 proteids and the* phosphorus of the fats are insignificant in amount ; 



532 



