CHAPTER VIII 



EXCRETION 



MANY things enter into the alimentary canal. If an analysis 

 were made of a day's food and drink, from the cup of tea on 

 waking to the cocoa or other potion which is regarded as a 

 necessary preliminary to settling for the night, it would be 

 found that a great variety of substances were included in the 

 food or taken as adjuvants to food. All these things, differing 

 widely in chemical constitution, must leave the body. Some 

 are not digested. They do not, properly speaking, enter into 

 the diet. Such are the cellulose of vegetables, especially skins, 

 husks, woody fibres ; elastic fibres of meat ; horny substances, 

 etc. The quantity varies greatly, according to the nature of 

 the diet. About 2 ounces (weighed dry) is the average. With 

 this indigestible refuse is included undigested food, if the diet 

 be excessive, and a variety of substances secreted by the liver, 

 such as cholesterin and bile-pigment, some residues of the 

 secretions of the alimentary canal, and products of bacteric 

 fermentations. All food which is digested and absorbed is 

 oxidized. It leaves the body by the lungs, the kidneys, or the 

 skin. Foods, as already stated, are classified as proteins, 

 carbohydrates, and fats. The chief excreta are carbonic acid, 

 water, and urea. Carbonic acid makes its exit from the lungs ; 

 water from the lungs, the kidneys, and the skin ; urea from the 

 kidneys. The three great groups of foods and the three great 

 groups of excreta overshadow in amount all the other sub- 

 stances which pass through the system. A balance-sheet in 

 which proteins, carbohydrates, and fats appear on one side, 

 carbonic acid, water, and urea on the other, is substantially 

 correct. The energy which is set free by burning in a calori- 

 meter the items entered on the debit side, after deducting that 

 yielded by burning the urea (carbonic acid and water are in- 



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