Such a tubule, viewed as a hydrostatic mechanism, presents three portions, evidently fitted for different functions: (1) The glomerulus is an apparatus which allows of the rapid exudation of water from blood. (2) The contorted portions of the tubule present the appearance of a secreting mechanism. The large soft, cloudy cells which line them are eminently fitted to take from the blood, or rather from the lymph which fills the tissue-spaces which intervene between the walls of the capillary bloodvessels and tubules, the various substances which they excrete. (3) The loop of Henle is a remarkable piece of apparatus, the purpose of which has been a subject of much controversy. Looking at it from the point of view of hydrostatics, it seems safe to conclude, from its extremely narrow bore, that it raises the pressure of the fluid in the glomerulus and first contorted portion; but it may have other functions also. A consideration of the arrangement of the bloodvessels of the kidney bears out the conclusion that the secreting apparatus is divisible into at least two separate portions, possibly into three. The glomeruli are supplied by short and relatively wide arterioles. Each arteriole breaks up, as soon as it enters the capsule, into a bunch of capillary vessels, which, in the same abrupt manner, reunite to form a venule. On leaving the capsule, this little vein behaves in a fashion for which the only parallel is to be found in the portal system of the liver. Instead of uniting with a larger vein, it again breaks up into capillary vessels, which supply the contorted tubules and loops of Henle. The medulla of the kidney is supplied by long arterial capillaries of the usual type. The short arterioles of the glomeruli are controlled by nerves which, constricting them, or allowing them to dilate—possibly by actively causing them to dilate—rapidly diminish or increase the amount of blood passing through their tufts of capillary vessels. Here, therefore, is a mechanism by which the glomeruli can be suddenly flushed with blood—a condition favourable to exudation into the urinary tubules. The interposition of a second set of capillaries prevents this sudden flushing from unduly disturbing the pressure in the vascular system as a whole. In the renal-portal capillaries of the kidney the blood-pressure is fairly constant and, presumably, low. The use of the term “renal-portal” is justifiable, not only on the ground that the vessels of the kidney behave like those of the portal system of the liver, but also owing to the very significant fact that in fishes and amphibia the kidney actually has a double blood-supply. In such an animal as the frog the glomeruli are supplied with arterial, the tubules with venous, blood. The glomeruli receive branches from the renal artery, the tubules from a portal system derived from veins of the abdomen and hind-legs.