The kidneys are so arranged by their anatomical structure--that of the cortical and medullary substance, the tubuli urineferi, pyramids, malpighian bodies, etc.--that they separate from the blood the solids in a state of solution. The secretion takes place by the agency of the gland cells, and equally in all the parts of the urine tubes. The protoplasmic cells which line at least a large portion of the tubuli urineferi elaborate from the blood certain substances, and discharge them into the channels of the tubules. All parts of the tubular system of the kidney take part in the secretion of urine as a whole, but there is another provision of vessels for a more simple draining off of the water from the blood when required. The large size of the renal arteries and veins permits so rapid a transit of the blood through the kidneys that the whole of the blood is purified by them. The secretion of urine is rapid in comparison with other secretions, and as each portion is secreted, it propels that which is already in the tubes onwards into the pelvis of the kidney. Thence, through the ureter, the urine passes into the bladder, into which its rate and mode of entrance has been watched. The urine does not enter the bladder at any regular rate, nor is there a synchronism in its movement through the two ureters. In a recumbent posture the urine collects for a little time in the ureters, then flows gently, and if the body is raised, runs from them in a stream till they are empty. Its flow is increased in deep inspiration, or straining, and in active exercise, and in fifteen or twenty minutes after meals. Substances taken into the stomach pass very rapidly through the circulation. It does not take longer than one minute for ferrocyanide of potassium to pass through. Vegetable substances pass in from sixteen to thirty-five. Neutral alkaline salts with vegetable acids, which were generally decomposed in transitu, made the urine alkaline in twenty-eight to forty-seven minutes. But the time of passage varied much; and the transit was always slow when the substances were taken during digestion. There are really two distinct parts in the kidney--the actively secreting part, the epithelium of the secreting tubules; and what maybe called a filtering part, the malpighian bodies. The specific gravity of urine is 1020--that is, the average human urine. Urine varies--in the morning before breakfast it is darker, urina sanguinis; urine secreted shortly after the introduction of any considerable quantity of fluid into the body, urina potus; and the urine evacuated immediately succeeding a solid meal of food, urina cibi. The last kind contains a larger quantity of solid matter than either of the others, the first and second being largely diluted with water.