Polyuria is the name of a symptom the presence of which may be easily ascertained beyond a doubt, but which is notwithstanding occasionally overlooked. Its existence is to be determined by measuring the urine. In extreme cases this may be unnecessary, but slighter forms may easily escape notice if this is not done. The quantity of urine normally secreted varies considerably, owing to many causes, of which the principal are--the quantity of fluid ingested, not necessarily in the form of beverages, but of food more or less succulent; the activity of the other secretions, especially those of the skin and the intestines, and the presence of substances which increase the rapidity of its flow through the kidney or stimulate the glandular cells; and, to a certain extent also, individual peculiarities. The quantity of water furnished by the kidneys depends largely upon the excess of pressure in the vessels, and especially in the Malpighian coils, over that in the interior of the tubes, and is consequently influenced by the general blood-tension. The second factor of importance is the calibre of the renal vessels, especially the arterioles; and the third, the freedom of exit of the formed secretion from the uriniferous tubes. A certain amount of back pressure, so far from diminishing the amount of urine, seems to increase it, as shown in some of the cases of surgical polyuria, where the normal amount is considerably exceeded, while the renal parenchyma is being gradually destroyed. The arterioles of the kidney being, like all other arterioles in the body, under the control of the nervous system through the vaso-motor nerves, it is easy to see how the various affections of this controlling element may act upon the secretion of urine; neither is it possible to deny (although by far the most important factor in the rapidity of the urinary secretion has been shown to be the blood-pressure) that the nervous system may have a direct effect upon the secreting renal parenchyma. The normal quantity of urine for an adult of medium height and weight and ordinary habits as regards the ingestion of liquids may be stated as fifty fluidounces, or a liter and a half, which is of course to be considered as only a very rough approximation. One liter on the one hand, and two liters on the other, can hardly be considered pathological limits, unless the increase or decrease takes place under circumstances which ought to produce the opposite effect.