Even in the diuretic action of digitalis we see indications of something more than an alteration of the hydrostatics of the blood-supply of the kidney. The brisker circulation carries waste products to the liver; the liver transforms nitrogenous refuse into urea; urea stimulates the renal epithelium. It would be a mistake to lay too much stress upon the direct effect of the drug upon the blood-pressure in the kidney. Other illustrations throw the mere hydrostatics of the problem into the background. Adrenalin (extract of suprarenal capsule) causes a severe contraction of the small arteries, which raises the general blood-pressure considerably; but the increased blood-pressure is not accompanied by diuresis, because the glomerular arterioles share to a full extent, perhaps to a disproportionate extent, in the general constriction. In migraine and certain other disorders it frequently happens that the blood-pressure in the aorta is unduly high, yet very little fluid enters the renal tubules. If a “saline diuretic,” potassic nitrate, sodic acetate, or some other drug of the same kind, be administered, a copious flow is established, the blood-pressure is relieved, the distressing symptoms disappear. Then, again, certain diuretics, such as “sweet spirits of nitre,” tea, gin, etc., may bring about a flow out of all proportion to the alteration they produce in the hydrostatics of the circulation. The diuretic action of these various drugs is clearly due to increase in permeability of the renal epithelium. And, of all stimulants to secretion, urea, the natural stimulant, is the most effective. If a kidney be removed from the body, a cannula inserted into its artery, and defibrinated blood caused to circulate under pressure through the organ, water may or may not drip from the ureter. On addition of urea to the blood, a copious excretion is set up. In explaining the mode of working of the kidney, as, indeed, in explaining that of every other organ of the body, the mechanical aspects of the problem must be kept in the background. When we are contemplating the plan of construction of the kidney, the hydrostatics of the circulation attract attention; but alterations in hydrostatic conditions are not the initiating cause of a greater or less flow of urine. The chemical condition of the blood circulating through the kidney is the initiating cause. When the presence in it of urea demands a more copious flow, the hydrostatic conditions are adjusted to this need. In the case just cited of the isolated kidney, it might be urged that the flow caused by urea is a mechanical effect. The cells of the contorted portions of the urinary tubules remove urea from the blood. They secrete it into the tubules. The solution of urea, being headed up towards the glomeruli, owing to the resistance offered to its passage down the tubules by the narrow, descending limbs of Henle’s loops, surrounds the capillary tuft. Urea rapidly attracts water from the blood.