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Calculus. Concretions of solid matter sometimes develop within the body; some constituent part of a secretory or excretory fluid, whether from being present in undue quantity, or from some other cause, fails to be eliminated in the dissolved condition, and gradually accumulating in the solid form constitutes a calculus or stone. Thus the ducts of the salivary glands may be blocked by a salivary calculus, concretions may form in the gall bladder constituting biliary calculi or gall stones, and last, but by no means least, a urinary calculus may develop either in the kidney or in the bladder.

Urinary calculi may be composed of several different substances. The stone may be originally developed in the bladder, and in that case is usually composed of triple phosphate (ammonio-magnesian phosphate), layers of which substance are deposited as the result of alkaline fermentation in the urine. Or the stone may in the first place form in the kidney, and subsequently descend into the bladder; such calculi are usually made up of uric acid (or urates) or oxalate of lime. The two last-named substances are rendered insoluble by undue acidity of the urine, while triple phosphate is deposited, as already indicated, as a consequence of undue alkalinity. It is thus easy to understand how it comes about that a urinary calculus so often consists of superimposed layers of differing chemical composition. The nucleus of the stone consists, for example, of uric acid, formed in the kidney as the result of undue acidity of urine; after a time the calculus passes down the ureter and reaches the bladder, there it sets up inflammation (cystitis), and, as a consequence of this, the bladder contents become alkaline. This changed reaction causes deposition of phosphates which accumulate, forming a layer external to the nucleus of the stone, and thus what is called an "alternating calculus" is produced. Urinary calculi may be formed in rare instances of other substances, e.g. cystin, xanthin, carbonate of lime. The causation of stone in the bladder is enveloped in considerable mystery. The deposit of layers of mixed phosphates, consequent upon the inflammation in cystitis, is, of course, well understood, but it is by no means so clear why the uric acid and oxalate of lime calculi are formed. Stone in the bladder is more common in men than in women, and more usually met with at the extremes of life than in people of middle age. It is certainly associated with locality; in parts of India, for example, calculus is of common occurrence. The symptoms are pain, increased frequency of micturition, and the passage of blood in the urine. The pain is especially felt, as a rule, at the end of micturition, when the wall of the bladder contracts upon the calculus; in some instances but little pain may be experienced, particularly if the calculus be large. The advent of cystitis brings with it a fresh group of symptoms, and the kidneys themselves may later become involved as the result of the bladder mischief. The presence of a calculus being suspected by the surgeon, he proceeds to explore the bladder by means of a sound. This instrument is it metal rod of suitable shape, which is passed down the urethra, so that one end projects into the bladder, while the other is held between the surgeon's fingers. Contact between the stone and the end of the sound, striking the stone as it is called, is the only indubitable evidence of vesical calculus. In the treatment of urinary calculus much has been thought and written on the subject of solvents. Practically, when a stone has once formed the only cure is its removal by surgical operation. Either the bladder is opened, or the stone is crushed in the bladder, and the fragments washed out and so removed.

Gall stones are usually composed of cholesterin or of bile pigment. They occur most commonly in women of middle age, but their mode of origin is ill understood. Gall stone colic is caused, as a rule, by the expulsion of the calculus from the gall bladder. The stone may reach the duodenum, and travelling down the intestinal canal, be removed from the body; or it may set up inflammation and give rise to serious trouble. Gall stones are sometimes removed from the gall bladder by surgical operation.