diffuses from lymph, through the walls of the capillary vessels, into blood, from which it passes into the air in the lungs. 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 substances 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 calorimeter the items entered on the debit side, after deducting that yielded by burning the urea (carbonic acid and water are incapable of further oxidation), gives a day’s income. Other constituents of the diet are so small in quantity as to be negligible in making up the body’s accounts. The chemical changes which they undergo add practically nothing to its capacity for work. Yet some of them are essential to the maintenance of health. Of such are common salt (sodic chloride), alkaline and earthy carbonates, sulphur, phosphorus, etc. These things, together with some products of action of the bacteria in the alimentary canal, the final stage of hæmoglobin, imperfectly oxidized nitrogenous substances, and other soluble substances which enter with, or are formed from the food, are removed by the kidneys. We speak of the elimination of waste products, as excretion. Not that there is any physiological distinction between excretion and secretion. Both terms refer to the selection or production and the discharge of materials by cells. If the product discharged has a useful function to perform—if it be a digestive