VI STATISTICS OF NUTRITION 269 



The waste which the body excretes and its distribution 

 among the chief excretory organs is shown in the table on 

 the following page, in which the ".water and other 

 matters " represent the total waste. 



The "other matter" from the lungs is chiefly carbonic 

 acid, in which the larger part of the carbon is excreted, 

 bringing with it nearly all the oxygen originally taken in 

 by the lungs. From the kidneys it includes urea, which 

 contains nearly the whole of the nitrogen excx'eted, 

 "together with some 25 grammes (nearly 1 oz.)of inorganic 

 salts. From the skin the "other matter" is a small 

 amount of salts and some carbonic acid, and in the faeces 

 it includes some 5 grammes of salts. The total output of 

 salts from the body is thus about 30 grammes (or rather 

 more than 1 oz.). 



This daily loss has to be made good by the new food 

 supplied. But in calculating the amount of material 

 necessary to replace the waste, we need only turn our 

 attention to the nitrogen and ,the carbon, for the water 

 lost represents almost entirely water taken as drink or in 

 the food, although a small amount comes from the oxida- 

 tion of the hydrogen of the food ; the oxygen is derived 

 from the air, and the salts are largely, though not entirely, 

 introduced as salts with the food. 



The daily waste of nitrogen and carbon may be taken 

 in round numbers as about 20 grammes (300 grains) of 

 the former and 270 grammes (or about 9| oz.) of the latter. 

 The niti'ogen necessary to make good this loss can only be 

 obtained from proteins. 



The necessity of constantly renewing the supply of 

 protein matter arises from the circumstance that whether 

 the body is fed or not, a breaking down of protein mate- 

 rial is continually going on, giving rise to a constant 

 nitrogenous waste, which leaves the body in the form of 

 urea. Now, this nitrogenous waste, coming from the 

 breaking down of protein material, can only be met by 



