406 METABOLISM, NUTRITION, AND DIET 



WEIGHT, COMPOSITION, AND HEAT OF COMBUSTION OF FOODS AND EXCRETA PER DAY. 



Careful analyses of the excreta, many of which we have already had oc- 

 casion to call attention to, show that they are made up, besides water, chiefly 

 of the chemical elements carbon, hydrogen, oxygen, and nitrogen, but that 

 they also contain, to a less extent, sulphur, phosphorus, chlorine, potassium, 

 sodium, calcium, magnesium, iron, and certain other of the elements. Since 

 this is the case it must be evident that, to balance this waste, foods must be 

 supplied containing all these elements to a certain degree, and some of 

 them in large amount, viz., those which take a principal part in forming 

 the excreta. 



The waste products of the body are eliminated through the lungs, the 

 skin, the alimentary canal, and the kidney. In the lungs the chief waste prod- 

 uct is water, carbonic-acid gas, and traces of ammonia compounds which 

 are composed of the elements carbon, oxygen, nitrogen, and hydrogen. Traces 

 of carbonic-acid gas and small quantities of urea and salts are eliminated 

 through the skin. From the alimentary canal there are lost, through the feces, 

 the indigestible and unabsorbed substances from the food, together with 

 products secreted into the canal by the liver, pancreas, and mucous membrane. 



