684 NUTRITION portion of all glandular organs, consists of tubes of base ment membrane lined with cells of peculiar attributes and surrounded by capillaries for blood and lymph, which allow their fluids to come into close communication with the secreting cells. The complex disposition of the tubes, of which there are hundreds in each kidney, has been traced after an infinite amount of patient research. An account of the arrangement of the tubuli uriniferi falls beyond the scope of the present article. Excretion of Urine. A review of the constituents of the urine discloses that the function of the kidneys is to separate from the blood chiefly (1) nitrogenous crystalline bodies which are undoubtedly the end-products of the oxidation of nitrogenous bodies, and (2) inorganic salts and water. We have now to describe the probable manner in which this separation is effected. Ever since the whole course and form of the renal tubules became mapped out, and the existence of a double system of capillaries was established, it has been the habit of physiologists to regard the excretion of urine as a twofold operation. Sir William Bowman, so long ago as 1842, in the course of a histo- logical investigation of the structure of the kidney, came to the conclusion that the watery portions of the urine are excreted in the capsule, while the solid parts are removed from the blood surrounding the lower parts of the renal tubule. Ludwig, about the same time, advanced the theory that the whole of the urine is separated from the blood in the glomeruli, but that it is separated in an extremely watery condition, the object of the complicated renal tubules being to permit of the reabsorption of the water, and to bring the urine into a suitable state of concentration for removal from the body. The progress of investigation has completely vindicated the theory proposed by Bowman, which now rests not merely on inference from anatomical structure, but upon a sound basis of physiological facts. Not only is the process twofold in respect of the two classes of constituents secreted ; it may be twofold also in respect of the processes of the act. There is at least much reason for thinking that the water is separated from the blood mainly, though not entirely, by a physical pro cess of filtration, while some, if not all, of the specific elements of the urine are secreted by a peculiar selective or elaborative action of living epithelial cells. The reason for supposing that the excretion of Avater is mainly a pro cess of filtration is the simple one that the flow of urine seems, in a general way, to obey the same laAvs of pressure as the flow of water through a filter. For example, what ever tends to increase the blood-pressure in the branches of the renal artery tends to increase the flow of urine in a given time. If the heart beats more quickly than usual more urine is excreted. If cold contracts the superficial vessels of the skin, and drives a larger quantity of blood upon the kidney, or if the same result ensues from moderate stimulation of the splanchnic nerves, the blood-pressure in the renal artery becomes raised, and an enlarged secretion follows. If the spinal cord be divided in the lower part of the cervical region, the great fall of blood -pressure which results is associated with suppression of the excre tion of urine. Further, if the pressure in the ureters be allowed to reach the value of 10 to 40 mm. ( 3937 to 1*5748 inches) of mercury the transfusion of fluid from the blood is prevented. Now the blood-pressure in the renal artery is about 120 to 140 mm. (47244 to 5-5118 inches) of mercury. Secretion stops, therefore, long before the pressure in the glandular ducts reaches that of the glandular vessels, a relationship between pressure and secretion the very reverse of that which obtains in the case of the salivary gland. These facts sufficiently justify the conclusion that the secretion of water in the kidney is a process of filtration. But there are other facts which demand the assumption that even the separation of water is in part an act of living protoplasm. The free inges- tion of water by drinking is one of the most certain ways of producing a copious flow of urine; and yet there is no evidence to show that in most cases this effect is brought about by a raising of the blood -pressure. On the contrary, if one animal be bled into the veins of another there is no increase produced in the amount of urine excreted, and it is difficult to imagine that the blood- pressure may be more easily raised by the drinking of water, however freely, than by introducing into the blood vessels of one animal the quantity of blood proper to two. In such cases we must assume that the outflow of water from the blood is due to the activity of living cells aiding the normal influence of pressure. While the secretion of water cannot wholly be ascribed to physical processes, there is no doubt that the secretion of some other bodies is a selective act of living protoplasm taking place independently of, or it may be in opposition to, physical processes. The first substance whose secretion was experimentally proved to be due to the renal epithe lium was sulphindigotate of soda, or indigo-carmine. If this substance be injected into the blood-vessels of an animal in which the blood-pressure has been so far lowered by division of the spinal cord below the medulla that the flow of urine has stopped altogether, it can be traced in a short time into the epithelium of the renal tubules, and through them into the lumen of the duct. The absence of the usual flow of water has left the granules stranded at the place where they entered the renal system of tubes. No trace of the substance was found by the original ob server, Heidenhain, in the glomeruli or capsules ; but it may be stated that later experimenters have succeeded, by injecting the drug freely and pursuing it over longer inter vals, in tracing it into the glomerular cells also. Sulphindigotate of soda is not a normal constituent of the urine ; but there is little doubt that what happens in the case of this body happens also in the case of some of the usual constituents. Thus, uric acid has been detected within the epithelium of the renal tubules; and by a com paratively new and beautiful observation it is placed beyond doubt that urea also enters the secretion at the same cells rather than through the glomeruli. This proof was rendered practicable by the discovery that in the frog the kidney is supplied with blood from two separate and distinct sources. The renal artery supplies the glomeruli with blood, while the so-called renal portal vein, a branch from the femoral vein which runs along the outer border of the kidney, supplies the capillary network surrounding the uriniferous tubules. If the renal artery be tied and the blood-supply shut off from the glomeruli, sugar when introduced into the circulation is not excreted with urine, although it readily is when the vascular supply of the kid ney is untouched. But in the same circumstances, when the action of the glomeruli is eliminated, urea is readily excreted. Urea and sugar are therefore removed from the blood by different organs ; or at least the renal epithelium of the tubules can excrete urea, although it cannot excrete sugar. At the same time that urea is being excreted a copious flow of water is determined, which most presum ably escapes at the point where the urea is excreted. This fact is significant when we remember that ligature of the renal artery usually stops the flow of water, and that sul phindigotate of soda injected into the circulation when the glomeruli are tied out of it may be traced into the same epithelium, but gives rise to no flow into the bladder. The renal epithelium, then, has the power of attracting to itself and removing from the blood certain elements which the latter already contains. It may now be asked whether the kidney has the power of elaborating any of
