*Urea* is the most abundant solid constituent of the urine and is the chief waste product arising from the oxidation of nitrogenous substances in the body. Although secreted by the cells lining the uriniferous tubules, it is not formed in the kidneys. The secreting cells simply separate it from the blood where it already exists. The muscles also have been suggested as a likely source of urea, for here the proteids are broken down in largest quantities; but the muscles produce little if any urea. Its production has been found to be the _work of the liver_. In the muscular tissue, and in the other tissues as well, the proteids are reduced to a lower order of compounds, such as the compounds of ammonia, which pass into the blood and are then taken up by the liver. By the action of the liver cells these are converted into urea and this is turned back into the blood. From the blood the urea is separated by the secreting cells of the kidneys. *Work of the Liver.*—The liver, already described as an organ of digestion (page 152), assists in the work of excretion both by changing waste nitrogenous compounds into urea and by removing from the blood the wastes found in the bile. While the chief work of the liver is perhaps not that of excretion, its functions may here be summarized. The liver is, first of all, a _manufacturing organ_, producing, as we have seen, three distinct products—bile, glycogen, and urea. On account of the nature of the urea and the bile, the liver is properly classed as an _excretory organ_; but in the formation of the glycogen it plays the part of a _storage organ_. Then, on account of the use made of the bile after it is passed into the food canal, the liver is also classed as a _digestive organ_. These different functions make of the liver an organ of the first importance. *Excretory Work of the Food Canal.*—The glands connected with the food canal, other than the liver, while secreting liquids that aid in digestion, also separate waste materials from the blood. These are passed into the canal, whence they leave the body with the undigested portions of the food and the waste from the liver. Though the nature and quantity of the materials removed by these glands have not been fully determined, recent investigations have tended to enhance the importance attached to this mode of excretion. *The Perspiratory Glands.*—The perspiratory, or sweat, glands are located in the skin. They belong to the type of simple tubular glands and are very numerous over the entire surface of the body. A typical sweat gland consists of a tube which, starting at the surface of the cuticle, penetrates to the under portion of the true skin and there forms a ball-shaped coil. The coiled extremity, which forms the secreting portion, is lined with secreting cells and surrounded by a network of capillaries. The portion of the tube passing from the coil to the surface serves as a duct (Figs. 91 and 121). [Fig. 91]