*The Necessity for Excretion* is found in the results attending oxidation and other chemical changes at the cells (page 107). Through these changes large quantities of materials are produced that can no longer take any part in the vital processes. They correspond to the ashes and gases of ordinary combustion and form wastes that must be removed. The most important of these substances, as already noted (page 110), are carbon dioxide, water, and urea.(74) A number of mineral salts are also to be included with the waste materials. Some of these are formed in the body, while others, like common salt, enter as a part of the food. They are solids, but, like the urea, leave the body dissolved in water. Waste products, if left in the body, interfere with its work (some of, them being poisons), and if allowed to accumulate, cause death. Their removal, therefore, is as important as the introduction of food and oxygen into the body. The most important of the excretory glands are *The Kidneys.*—The kidneys are two bean-shaped glands, situated in the back and upper portion of the abdominal cavity, one on each side of the spinal column. They weigh from four to six ounces each, and lie between the abdominal wall and the peritoneum. Two large arteries from the aorta, called the _renal arteries_, supply them with blood, and they are connected with the inferior vena cava by the _renal veins_. They remove from the blood an exceedingly complex liquid, called the _urine_, the principal constituents of which are water, salts of different kinds, coloring matter, and urea. The kidneys pass their secretion by two slender tubes, the _ureters_, to a reservoir called the _bladder_ (Fig. 87). [Fig. 87] Fig. 87—*Relations of the kidneys.* (Back view.) 1. The kidneys. 2. Ureters. 3. Bladder. 4. Aorta. 5. Inferior vena cava. 6. Renal arteries. 7. Renal veins. *Structure of the Kidneys.*—Each kidney is a compound tubular gland and is composed chiefly of the parts concerned in secretion. The ureter serves as a duct for removing the secretion, while the blood supplies the materials from which the secretion is formed. On making a longitudinal section of the kidney, the upper end of the ureter is found to expand into a basin-like enlargement which is embedded in the concave side of the kidney. The cavity within this enlargement is called the _pelvis of the kidney_, and into it project a number of cone-shaped elevations from the kidney substance, called the _pyramids_ (Fig. 88).