The kidneys excrete and put the finishing touches upon the urinary fluid; they act at the end of the metabolic course, both as active and as passive agents. While to some extent the kidneys are mechanical contrivances, mere filters, so to speak, they are to a far greater degree active, specific glands. The renal epithelium has the power of synthesis, and builds up complex substances that are not apparent as such in the blood or lymph, as well as the power of analysis or breaking down of substances. These changes are due to the formation in the tubular cell of an enzyme, which we call histozyme. The kidney is the end link in the vascular chain, and the daily performance of healthy kidneys is no doubt a combination of the mechanical and the vital processes. _The Secretion of the Urine._—In a perfectly normal being the problems of waste and repair are balanced to a nicety. As we know, the tissues of the body are bathed in lymph containing in solution the compounds that are necessary for their nourishment—proteids, carbohydrates, fats, salts, and gases. Waste follows in direct proportion to the activity of the tissues. The worn-out, effete material first finds its way into the lymph, and from it into the blood-stream, to be later eliminated from the economy, else deleterious results will follow their retention in the body. It is by the selective action of the cells of the various glands of the body that these useless substances are removed from the blood, and converted into such forms as can be readily excreted. In the main, the products to be removed are urea and the allied nitrogenous bodies—carbon dioxid, salts, and water. These organs are of vital importance, since nearly all of the waste-products containing nitrogen are eliminated in the urine. The most easily understood function of the kidneys is the excretion of the urinary water; this varies in amount with the rapidity of flow through the renal vessels and to some extent on the blood-pressure. [Illustration: Fig. 11.—Relation of kidneys to heart and great blood-vessels: A, Heart; B, B, kidneys; C, bladder (after Quain).] The separation of the solid substances of the urine form the next function of the kidneys, and these substances fall into two groups—inherently useful materials, which are in excess or which have served their purpose, and substances which are inherently harmful. The latter class embrace many end-products of metabolism, which we are accustomed to consider as normal constituents of the urine. The analytic study of the urine is of great value to the physician and surgeon, because of the knowledge which it gives concerning the processes of metabolism occurring within the body. The nature and amounts of the various end-products of metabolism are carefully investigated as they occur in the urine, whether they be normal or pathologic.