It also contains sweat glands, which throw off body waste in the form of salts and moisture in the perspiration; this helps to regulate the body heat and to aid in keeping the skin soft. The kidneys and the skin are interdependent; if the kidneys are inactive the skin must throw off a larger quantity of waste and if the skin is inactive, or if for any reason its pores are closed, the kidneys become more active. The skin also throws off carbon dioxid and, to a slight extent, it absorbs oxygen. Besides digesting and absorbing food, the _intestines_ eliminate waste. In their work of elimination, they pass off all undigested matter. They also carry off bile pigment, bile salts, mucus, other decomposition products—also a little unabsorbed fat. Coarse articles of food containing fibers which do not digest, such as the bran of grains and the coarser fibers of fruits and vegetables (much of their substances are not food in the strictest sense), are valuable, as they increase the peristaltic movements of the intestines and assist in carrying the waste excretions along their course. The intestines also carry off the organic refuse which is produced by the chemical action of oxygen. This refuse consists of carbon dioxid and the nitrogenous waste. Combustion, or burning of fuel in any form (oxidation to release latent heat and energy), always leaves a residue, and it is the work of the intestines to eliminate much of this refuse. When coal is burned, gas, smoke, ashes, and cinders constitute the waste; if these were not allowed to escape or were not removed from a stove the fire would soon go out—the smoke and gas would smother it and the accumulation of ashes would prevent the circulation of oxygen. This is true in the body—the carbon dioxid not being allowed to pass off would soon put out the fires of life; it would poison the body and inhibit the action of the nerves. If the waste is not thrown from the system we notice it in a feeling of lassitude, both mental and physical. If the nitrogenous waste (like ashes and cinders) is not eliminated, one will die in convulsions in a few days. The absolute necessity of a free elimination of waste will be readily seen. If the engine is to do its work, the engineer sees that it is kept perfectly clean—otherwise it becomes clogged, works inefficiently, and soon wears out. The same is true in the body—clogging in any part overworks and wears out other parts dependent on the work of the defective one. Constipation, or a failure of the intestines to eliminate the waste is a grave menace to the system. The poisonous gases accumulating are absorbed by the system. The _blood_ carries the digested food and the oxygen to the various tissues and organs, which select from among the nutrients offered to them the ones suited to their growth and repair. It is the universal medium of exchange. It carries carbon dioxid to the lungs and the wastes of the tissues to the other eliminative organs.