=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. [Illustration: FIG. 59.--A longitudinal section of the kidney. (Leroy.) _a_, Renal artery; _c_, cortex; _m_, medulla; _u_, ureter.] [Illustration: FIG. 60.--A Malpighian body or corpuscle. (Leidy.) _a_, Afferent artery; _e_, efferent vessel; _c_, capillaries; _k_, commencement of uriniferous tubule; _h_, uriniferous tubule.] _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_.