About forty, or from thirty to fifty, fluid ounces (a quart, more or less) of urine is passed by a healthy grown person every twenty-four hours. It may be retained longer in the female than in the male bladder, but not many hours commonly in either. More is passed, and more frequently, during winter than in summer.

The color of healthy urine is that of amber. It should be clear when passed, and should have very little settling at the bottom, even after standing for some hours. Yet some change in color, lighter or darker, or variations in quantity, and even deposit of sediment, may take place while the person continues in health. Such alterations often show the successful relief of the system, by excretion, of what, if not carried off, might have caused disease. Great and co?continued alterations in the urine, however, are important signs of something being wrong; and, under skilful examination, the nature of the disease may thus be found out. For this kind of inquiry the skill of the physician, trained in the use of chemical tests and the microscope, will be required.

Gravel is the term applied to small stony particles which are formed in the kidneys from disease, and pass, first along the ureters to the bladder, and thence out through the urethra with the flow of urine. Pain, sometimes very severe, may attend both of these short journeys of particles, if they be large. Often, they are more like sand than gravel, and escape without giving pain, except that both the kidneys and bladder are apt to be in a state of irritation at the time of an " attack of gravel."

Stone in the bladder is of the same nature, only the particles accumulate into one or more masses, which may become very large, and cause great suffering; not seldom, unless removed by an operation, shortening life.

Gall-Stones are formed by thickening of bile in the gall bladder, which lies under the liver, on the rightside, near the middle of the body. Although the gall-duct, through which such stones pass to the small intestine, is short, a large gall-stone (biliary calculus) sometimes gives extreme pain in its passage. Complete relief comes when it enters the small intestine {duodenum); as is the case likewise when a gravel-stone escapes from the ureter into the bladder.