The sweat glands also throw off an excess of water and salts. The kidneys and the skin are interdependent; if the kidneys are inactive the skin throws off a larger quantity and if the skin is inactive, or if for any reason the pores of the skin are closed, the kidneys are more active. This is evidenced by the sudden immersion of the body in cold water; the pores of the skin being closed the kidneys immediately act. During the summer, or at any time when the skin throws off more water than usual, the kidneys are less active and the urine, being more concentrated, is darker. The skin also throws off carbon dioxid and, to a slight extent, it absorbs oxygen. * * * * * [Sidenote: Work of the Intestines] The intestines, in their work of elimination, pass off all undigested matter. They also carry off bile pigment, bile salts, mucus, amino acids, and other decomposition of proteins,—also a little unabsorbed fats and bacterial decomposition taking place in the intestines. Coarse articles of food containing fibres which do not digest, such as the bran of grains and the coarser fruits and vegetables (though much of their substances are not food in the strictest sense) are valuable to increase the peristaltic movements of the intestines and to act as a carrying body to move the waste excretions along their course. The combustion, or burning of fuel in any form, (oxidation for the release of latent heat and energy) always leaves some parts which are not used as heat or energy, and it is the work of the intestines to eliminate much of this refuse. When coal is burned, gas, smoke and cinders or clinkers, constitute the waste and if these were not allowed to escape from a stove the fire would soon go out—the smoke and gas would smother it and the clinkers would prevent the circulation of oxygen and soon clog and fill the stove. The same is true in the body—the carbonic acid gas not being allowed to pass off would soon put out the fires of life; it would poison the body and stunt the action of the nerves. If the nitrogenous waste (like ashes and cinders) is not eliminated by the kidneys, one will die in convulsions in one or two days. The absolute necessity of a free elimination of waste will be readily seen.—If the engine is to do its best work, the engineer sees that it is kept perfectly clean—otherwise it becomes clogged, does inefficient work and the clogging soon wears out some parts. The same is true in the body,—clogging in any part overworks and wears out other parts dependent upon the work of the one. Summary Let us sum up the processes which the food undergoes in its conversion into condition to be absorbed by the body; in its absorption through the walls of the intestines and stomach; and the metabolic processes which it undergoes in being converted into heat and energy and again broken down and eliminated as waste.