ferment, for example—it is said to be secreted. If it is of no further use to the economy, we say that it is excreted—got rid of. In some cases either term is equally appropriate. The sebum prepared by the sebaceous glands is useful as a lubricant of the skin. It is thrown off. We may speak of the glands as either secreting or as excreting this fatty substance. =The Kidney.=—From worms upwards, all animals possess organs for the removal of waste products in solution. This statement might, indeed, be widened so as to include animals even lower than worms. All animals which have a cœlomic cavity—a space between the alimentary canal and the body-wall—have organs for the removal of soluble waste. The segmental organs of worms are obviously the same organs as the kidneys of mammals; the latter are distinguished from their prototypes by greater concentration of structure and specialization of function. The kidney is the oldest of organs, if its antiquity be estimated as the length of time during which it has had a form practically identical with that which it now presents. The lungs are of late appearance in the animal scale. Alimentary canal, heart, brain, have passed through many transformations. The kidney assumed its permanent form very far back in the history of the animal kingdom. The most primitive animal which has a digestive cavity, and vessels in which the products of digestion circulate, needs an organ which provides for the overflow from the body-fluids of all substances which are injurious or effete. The kidney is an aggregation of long urinary tubules. The head of each tubule is dilated into a globular capsule, into which a tuft of bloodvessels depends. This is the sink into which the waste-water of the blood drips. The long urinary tubules are lined with cells well qualified by form and constitution to search the blood in the capillaries which border them, for substances which, not being easily diffusible, have to be forcibly dragged from it and added to the water trickling down the pipe which connects the rain-water head with the sewer. The hydrostatic conditions of this apparatus—the provision for greater or less flow of blood through the tufts (glomeruli) which hang in the capsules, and for longer or shorter exposure of the blood to the purifying activity of the epithelium of the renal tubules—will be described after a very brief account has been given of the structure of the organ. The outer border of the kidney is convex, its inner border concave. The concavity is termed the “hilus.” The central depression of the hilus is embraced by the expanded end of the ureter—the tube which carries the secretion of the kidney to the bladder. The renal artery and the renal nerves enter, and the renal vein leaves, the kidney at the hilus.