The Kidneys.—The two kidneys lie on either side of the vertebræ at the back of the abdominal cavity and behind the peritoneum, between the last dorsal and the third lumbar vertebræ, their inner edge being about one inch from the spinous processes. They are bean-shaped, four inches long, two inches wide, and one inch thick, and are embedded in a mass of fat and loose areolar tissue. They can be felt only when misplaced or when enlarged, as by tuberculosis or malignant disease.

The whole kidney is enveloped in a fibrous capsule which normally may be peeled off but which in some diseases becomes adherent. On the internal border is a fissure or hilum, through which pass the blood-vessels and the ureter. Upon entering, the ureter dilates into a sac, the pelvis of the kidney, into which project the Malpighian pyramids of the medullary substance, a substance made up of the straight uriniferous tubules and blood-vessels. Outside the medullary substance and just under the capsule is the cortex, containing the Malpighian bodies, blood-vessels, and the convoluted tubules or loops of Henle. Each Malpighian body contains within a capsule a plexus of capillaries, the glomerulus, with an afferent arteriole and an efferent vein. The renal artery is a branch of the aorta and the nerves are from the solar plexus.

The Urine.—As the blood passes through the glomeruli, the urine is filtered off as it were, probably by a process of transudation rather than simple filtration. The cells lining the tubules also play an important part in its formation, not by secreting new substances but by taking up those brought by the blood and discharging them into the convoluted tubules, from which the urine passes through the straight tubules of the medulla to the pelvis, to be carried thence by the ureter. The process of the formation of the urine, therefore, is not purely a process of secretion but requires some action on the part of the kidney, though no new substances are secreted in the kidney.

The passage of the urine down through the ureters is assisted by a kind of peristaltic action in the walls of the ureters and it is expelled from the body by the act of micturition, which is mostly voluntary, though a certain amount of nervous mechanism controls it. The seat of this nervous mechanism is in the lumbar enlargement of the spinal cord. In some nervous conditions, especially where there is injury to the spinal cord, there is involuntary micturition.

The urine is a watery solution containing many waste products, especially urea. It is generally amber in color, varying in shade with circumstances, with an aromatic, characteristic odor when fresh. It is acid in reaction and has a specific gravity of about 1020, though this too varies with circumstances. Besides water, which is its chief constituent, it contains urea, uric acid, organic acids, urates, inorganic salts, including sodium chloride and phosphates of calcium and magnesium, a certain amount of ammonia, and certain pigments. Its acidity is due to acid sodium phosphate in solution but varies with the food, and in disease the urine may become alkaline when passed. After standing a few hours in a warm place it decomposes and becomes alkaline.

The quantity, which is normally three pints or fifteen hundred cubic centimeters in twenty-four hours, varies with the amount of fluid drunk, the amount of perspiration, etc. The amount secreted depends chiefly, however, upon the flow of the blood through the kidneys; the greater the flow of blood, the larger the amount of urine formed; and the blood flow is determined by blood pressure and by vasomotor action. Secretion also seems to be increased by the presence of urea, which apparently serves as a stimulant to the kidney cells.

The excretion of waste materials takes place by three main channels, the lungs, skin, and kidneys, and the materials are of four kinds, urea, carbon dioxide, salts, and water. The lungs carry off carbon dioxide and water chiefly, the skin these and inorganic salts, while the kidneys eliminate practically all the urea as well as inorganic salts and water. When the kidneys are not working the skin carries off much urea. In fact, a close relationship exists between the kidneys and the skin in the matter of excretion. Thus, with increased perspiration in warm weather comes decreased urine, while in cold weather the blood is sent in and the urine increased in amount.

To incite action of the kidneys drugs known as diuretics may be used. These act in two ways, by stimulating the kidney cells directly and by acting on the general circulation or nervous system. Any emotional or nervous excitement increases the flow of urine.

There are certain abnormal constituents of urine, of which the two most important are albumen and sugar. The former is found only when there is some disturbance of the kidneys, ureters, or bladder, and its presence usually denotes some change in the cells lining the urinary tract. It may occur in congestion of the kidney as well as in disease. Sugar is found only in diabetes, the amount varying with the severity of the disease. In jaundice certain bile pigments are present in the urine, giving it a dark brown color and to the foam a greenish-yellow color. Even normal urine has some sediment upon standing, consisting of cells from the urinary tract and mucus. In very acid urine after standing a heavy sediment, whitish or pinkish, i.e., brick dust, in color, is thrown down. It does not necessarily denote disease, but shows the urine is acid and concentrated. In alkaline urine there is a sediment due to phosphates.

Rupture of the kidney occurs but is not so serious as rupture of the liver or spleen because the kidney is situated outside of the peritoneum. It necessitates the removal of the kidney, however, and when for any reason one kidney is removed the other increases in size and does double work to compensate for the loss. Removal of both kidneys means death. Sometimes the kidney becomes loose and moves about, a condition known as floating kidney. Perinephritic abscess is abscess in the loose fatty tissue about the kidney.

The Ureters, one for each kidney, are tubes the size of a goose quill and about fourteen inches long, extending from the hilum of the kidney to the base of the bladder. They have three coats, an internal mucous, a muscular, and an external fibrous coat, this last being continuous with the cortex of the kidney and the fibrous tissue of the bladder. In the female the ureters may be felt through the wall of the vagina as they come into the bladder. In tubercular disease of one kidney the ureter becomes inflamed and enlarged and through the vagina feels almost like a lead pencil, a sure diagnostic sign.

The Bladder and Urethra.—In their course to the bladder the ureters pass from the abdominal into the pelvic cavity, but before describing the pelvis itself it will be well to complete the account of the urinary organs by considering the bladder and urethra. The bladder is the reservoir for the urine and has muscular walls lined with mucous membrane. A peritoneal coat covers the upper surface and is reflected to the walls of the abdomen and pelvis. It is situated back of the os pubis, the front bone of the pelvis, with its base or fundus directed downward and backward. Normally it is in the pelvis, but when much distended it mounds up into the abdominal cavity, where it can be felt in front as a tumor. It rests on the rectum in the male and on the cervix in the female and is held in place by numerous ligaments. When empty it may be Y-shaped, but it becomes oval when distended. Its capacity is about one pint.

The lower abdominal wall and the anterior wall of the bladder may be wanting congenitally. In paralysis of the sphincter at the neck of the bladder distention results. Stones may be found in the bladder.

From the neck of the bladder the urine passes out of the body through the urethra. This in the male passes down through the penis and is about ten inches long. Except when urine is passing it is a transverse slit with the upper and under surfaces in contact, while at the end of the penis the slit of the meatus urinarius is vertical. When the penis is flaccid, the urethra describes a sharp curve before its entrance into the bladder, but it becomes approximately straight when the penis is raised at right angles to the body—an important point to remember in catheterization.

In the female the urethra is straight and much shorter, being only about one and a half inches long. The meatus urinarius is in the anterior vaginal wall about one inch behind the clitoris.

Sometimes the urethra is ruptured in a fall. Stricture of the urethra occurs sometimes after gonorrhoea, owing to the formation of scar tissue following ulcer.