_The Urine._—The normal human urine recently passed is a clear liquid, of a straw color, with an average specific gravity of 1020, the specific gravity indicating the amount of solids contained in the urine. It is acid in reaction. The quantity for twenty-four hours is 50 ounces, or about 3 pints, depending on the amount of water ingested. During sleep the amount secreted is less than at other times. The amount of urine is decreased after profuse sweating, diarrhea, thirst, diminution in the blood-pressure, and after severe hemorrhage. When the body temperature is considerably higher than normal, the amount of urine is again diminished in quantity. Urea forms nearly one-half of the solids in normal urine and nearly one-half of the urea is nitrogen. It is the principal representative of the waste of the nitrogenous tissues. The chief source of urea is from the metabolism of the muscles; the ingestion of a large amount of proteid food stimulates metabolism. The quantity of urea passed in the twenty-four hours is about 500 grains. The uric-acid constituent is very small, and in the human urine scarcely reaches 0.03 per cent. of the solids. Lack of exercise leads to an increased formation of uric acid by a lessening of the oxidation of the tissues. In gout the amount of uric acid excreted is abnormally small, because it accumulates in the blood and tissues. The brick-dust deposit sometimes seen in the urine is composed chiefly of the urate of sodium. The average daily amount of uric acid passed in the human urine is about 7 grains. The excretion of uric acid may be increased by drinking copious drafts of water. _Toxicity of the Urine._—After the removal of both kidneys the animal dies of uremia; that is, there is an accumulation of urinary products in the blood. The removal of one kidney is not necessarily fatal, and women have so lived very comfortably for many years. A human being excretes enough poisonous material by the kidneys in two days to cause death. _The Urinary Excretory Apparatus._—After the urine has been secreted by the kidneys it must be carried away from the body, so that the economy may not suffer from the resorption of the contained toxic principles. The excretory apparatus comprises the ureters, the bladder, and the urethra. The ureters are two cylindric tubes of the diameter of a goose-quill and about 15 inches long. They make their exit from the inner border of the kidney, and pass downward and inward toward the median line, to empty into the base of the bladder by a slit-like orifice. The bladder is the reservoir for the urine. It is a musculomembranous sac, situated in the pelvis, and in the female it is in front of the uterus and above the vagina. It normally contains one pint. It should be emptied four times a day. If it is allowed to go longer than this, it becomes overdistended, and is apt to displace the uterus backward, and the bladder-walls themselves become weakened.