Calculi, stone-like concretions which form in different parts of the body, often about some undissolved particle in the fluid, which holds the matter of the concretion in solution, and again as a deposit upon some hard surface, as the tartar which collects upon the teeth. In the intestines the concretionary deposits are sometimes mechanical agglutinations of dry fibrous particles, as the fine down of the oat gathered about a piece of bone or stone of some fruit, and intermixed with layers of phosphate of lime. The fluids of the body may deposit concretions in most of the vessels, organs, and tissues. They are left by the blood in the arteries and valves about the heart; by the saliva in the mouth, in the substance of the cheek as well as upon the teeth; and by the bile in the gall bladder. They are found in the tissue of the lungs and in the bronchial glands, and in gouty persons under the skin, about the joints of the fingers and toes, etc. But their most common occurrence is in the kidney, bladder, and urinary passages, left by decomposition of the complex fluid of these organs. Urinary calculi are variously composed, and may be classed as those which are soluble in caustic potash or soda, and those which are insoluble. One of the most common of the former class is the uric acid calculus.

This ingredient in urine, when secreted in undue proportion, forms minute red crystals and red sand, which are passed in a solid state. If retained, they increase in size and produce the disease called the stone. The acid, if greatly in excess, is deposited in successive layers, forming yellowish-colored stones of such size that they can be removed only by the operation either of lithotomy, which is making an incision into the bladder and removing the stone by forceps, or of lithotrity, which is the introduction of an instrument into the urethra, by which the stone is broken, so that it may be removed by voiding it in fragments. If the uric acid is not in excess, the concretion once produced is liable to be covered with an incrustation of an ammonio^phosphate of magnesia or of a phosphate of lime, and thus increase in size. These phosphates, when deposited alone, as is sometimes the case, are included among the insoluble calculi, of which other varieties are produced in the form of oxalate of lime, called, from their resemblance to the mulberry, the mulberry calculus, of a brown color and mamelon-ated form, which are sometimes nuclei for the uric acid calculus; and again as carbonate of lime, which are of rare occurrence.

Other calculi, which belong to the soluble class, are formed with uric acid in combination with ammonia; others of cystic oxide or cystine, and of xanthic oxide or xanthine. These are distinguished from each other by their various shades of color, different degrees of hardness, and their peculiar reaction with different chemical agents. Concretions of uric acid are not uncommon with children, and recur in the same persons in advanced age. Those are most liable to them who suffer from dyspeptic and gouty tendencies. When this is observed, serious trouble may in most cases be obviated by particular attention to the diet, and by the use of proper medicines; but if the concretions are allowed to increase till they are too large to be passed, there is then no recourse but an operation; for, once formed, they are never afterward absorbed, nor has any solvent for them been discovered upon which dependence can be placed. - Calculi deposited by the bile in the gall bladder, the liver, and its ducts, are known as biliary concretions and as gall stones. They are generally of a round or oval form, and of various colors, as white, yellow, brown, and dark green.

Usually they are soft, and sometimes brittle and easily pulverized to an unctuous powder; their size has in some cases reached that of a walnut. In man they generally consist of cholesterine, more or less intermixed with the mucus and coloring matter of the bile; but some have been found consisting of carbonate of lime 72.7 per cent., phosphate of lime 13.51, and mucus 10.81. In animals their composition is very variable, some consisting of the same ingredients as are found in those of men. In the stomachs of ruminating animals they are found in the form of balls of hair, earthy matter, and food, cemented around some hard central nucleus.