of the vesicula prostatica, is cylindrical and almost indistinguishable from that of the urethra (Pl. I., A, 4 and 5, and B, 3).

The Female Urethra has no bulb; the mucous layer is very vascular, and is lined with pavement epithelium (Pl. I., B, 2 and 3). Only a small number of Littre's glands are found.

CHAPTER II.

THE EXCRETION OF THE URINE.

THE function of the kidneys consists in the secretion of the urine; that of the bladder and urinary ducts, in the gradual collection, retention, and discharge of the same. A perfectly satisfactory explanation of the secretion and excretion of the. urine in all its details is wanting.

Bowman advances the theory (in which he is supported by the anatomical construction of the kidneys) that the epithelial cells are the secretory organs, and that from the glomeruli only water escapes, which extracts the other constituents of the urine from the epithelial cells.

Ludwig bases his theory upon the varying bloodpressure in the renal vessels, and the interchange of constituents by osmosis through the animal membranes. He assumes that the pressure in the glomeruli is greater than in the capillary system immediately surrounding, and that consequently a profuse exudation of water occurs from the Malpighian tufts, which contains dissolved salts (also blood serum, a little albumen, and fat

globules). Accordingly, one finds in the urinary canals thin urine and in the surrounding capillaries thickened blood. These two fluids of such different densities, separated by a thin membrane, cause a ready osmosis, by means of which water from the urinary tubes enters the thickened blood; on the other hand, the urinary tubes receive from the blood the products of retrograde metamorphosis (urea and salts). In this manner the watery urine becomes more concentrated and richer in urea and salts-a true urine. The absence of albumen may be accounted for from the fact that it passes through animal membrane with much difficulty and only under great pressure. Under a pathologically heightened blood-pressure in the glomeruli (as stagnation of the renal venous system), one always finds albumen in the urine, but never under a physiological blood-pressure. Though this theory explains many physiological and pathological facts, it does not show how an acid urine can be secreted from an alkaline blood-serum. Hence the mechanical theory of Ludwig attributes to the glomerulus a process of filtration, and in the further course of the urinary tubes a process of osmosis, the office of the epithelium being wholly left out of consideration.

According to Goll and Max Hermann, the difference in pressure between the contents of the blood-vessels and the urinary tubules constitutes a chief motive power which forces the urinary constituents of the blood into the urinary tubules. Consequently if the blood-pressure in the renal artery is increased, then there is an increase

in the amount of urine secreted; but if the bloodpressure in the renal artery is diminished, or the pressure in the ureter approaches the normal blood-pressure in the artery, then is the secretion of urine lessened or made to cease entirely, long before the pressure in the ureter has reached the amount of pressure in the renal artery.

Ustimowitsch and Grützner have elaborated these theories to such an extent as to demonstrate, by experiments on the dog, that the secretion of urine is not dependent upon the general blood-pressure, but upon the local pressure in the glomeruli of the kidney. If the medulla oblongata of a dog be divided and the general blood-pressure be artificially increased by a current of electricity, the secretion of urine is prevented entirely, for the reason that the small vessels of the kidney be come contracted. If now the nerves of one kidney be divided, there ensues a profuse secretion of urine on that side, while no urine flows through the ureter of the other kidney. This is due to the fact that by section of the vaso-motor nerves of the kidneys the smallest arteries become expanded and relaxed, by which means the blood-pressure in these small vessels is increased and the secretion of urine brought about. Ustimowitsch also shows that by a diminution of the general blood-pressure an increase in the secretion of urine ensues. If one, for example, cuts through the sympathetic nerve, which contains the vaso-motor branches for the kidney, the blood-pressure in the aorta is reduced, but at the same time an expansion of the small

renal arteries occurs, causing an increase of the urinary secretion.

Heidenhein and Wittich support the theory of Bowman in regard to the secretory function of the epithelium; while they prove, by their experiments with indigo-sulphate of sodium, urate of sodium, and carminate of ammonium, that these substances become separated by the epithelial cells.

K. Müller's investigations show that the excretion of urine is increased by the application of cold to the skin, as fomentations or dressings; but by the application of heat, as in a warm bath, or by varnishing the skin, the excretion is diminished, the blood-vessels of the skin being dilated. A diminution of the quantity of blood in the skin capillaries increases the urinary secretion, while an increase of the former diminishes the latter.

According to Wendt, the increase of intra-abdominal pressure hinders the excretory process, probably by an increase of pressure in the renal veins, by which circumstance, as we know (Ludwig), the secretion of urine is checked.

Maly, Donath, and Posch have proved that, by the agency of osmosis, an acid fluid may be obtained from a watery solution of several salts (as mono- and di-sodic phosphates), which together give a neutral or slightly alkaline reaction with litmus. This is a discovery of importance, as it obviates the necessity of attributing to the renal epithelium the chemical property of acid formation.

Notwithstanding all these theories, there is no hypo