Officinal in two forms: (1) os ustum - bone-ash; (2) pure tribasic phosphate.

Preparation

(1) Os Ustum

When bones are calcined in close vessels, the residue consists of earthy salts mixed with charcoal (carbo animalis); but when calcined in open vessels, all animal and carbonaceous matter is burnt off, and the white friable residue consists mainly of phosphate and carbonate of lime (bone-earth, bone-ash). This, when treated with hydrochloric acid, and afterward with ammonia, is changed into (2) tribasic (or tricalcic) phosphate, Ca32(PO4), which is washed and dried at 212°, and forms a crystalline white powder, insoluble in water, soluble in acids. It has been found to contain lead (Duquesnel).

This form is the one most commonly found in nature, sometimes almost pure (phosphorite) or in friable masses, like chalk (osteoliths), or in the fossil faeces of ancient saurians (coprolites), in shells and sedimentary earths. From the soil it is absorbed by plants, by the help of water and carbonic acid, and is determined specially to the seed. From plants it is received by herbivorous animals, and in their flesh and blood and bone it is sought by the carnivora. It has been said that the amount of phosphate of lime found in different animals is proportionate to the activity of their movements (Dusart and Blache). (The salt was obtained formerly for medical use from the excrement of dogs when hard and white, as it is passed after they have eaten many bones; it was known as "album graecum.")

Besides the tribasic phosphate there are two others, a neutral and an acid phosphate. The former, Ca2H12(PO4), is a white, crystalline powder, tasteless and insoluble; it occurs in some (carbonated) mineral waters, and may be prepared by mixing neutral phosphate of soda with chloride of calcium. The acid phosphate, CaH12(PO4), is very soluble, and even deliquescent, and is left in solution when sulphate of lime is precipitated after treating bone-ash with sulphuric acid.

Absorption And Elimination

The various salts of lime differ somewhat as to their absorption and their action. The tribasic and neutral phosphates, in small doses (less than 5 or 6 gr.), with but little water, are wholly absorbed under the influence of the acid gastric secretion; but if given with much water, the acids are so far diluted that they do not act upon the insoluble drug, and it passes off mainly by the faeces. If large doses be given, the greater part passes out unchanged.

Gouriet has suggested that the solubility necessary for securing the absorption of lime phosphate is effected partly by means of the phosphate of soda contained in the saliva, partly by the phosphate of ammonia and the acids in the gastric juice; when it has passed into the veins, solubility is still further assisted by the carbonic acid present in venous blood. During respiratory combustion, when carbonic acid is given off and lactic and other acids altered, the phosphate that has been taken is only retained in solution by the help of the normal alkaline phosphates of the blood: if these be in small proportion the lime salts become soon deposited (more in bone than in other tissues), and little passes in the urine; if, however, in any given case the alkaline phosphates be in excess, then most of the lime salt is retained in solution in the blood until it is (mainly) excreted through the kidneys (Lancet, ii., 18G0, p. 251). This explanation seems rather too chemical, and it must be compared with the important observations more recently made by Paquelin and Jolly. They conclude that the tribasic phosphate of lime is not acted upon in the stomach, unless it be by part becoming super-phosphate, and this again is precipitated in the intestine under the influence of alkaline biliary and pancreatic secretions, as insoluble phosphate; it is not capable of absorption, except in very small quantities; the circulation conveys very little, and the tissues, except bones, contain only traces; the bile has rather more. A certain amount of lime must enter the system from the food, and does so mostly as carbonate, which becomes changed and prepared for absorption by contact with alkaline phosphates and gastric acids, but artificial phosphates are eliminated almost entirely unchanged, only some of the acid being absorbed. Hence they conclude that the addition of such compounds to the food is rather an obstacle to nutrition, and that even the soluble acid preparations (lacto-phosphates, etc.) act only as acid principles, and pass out of the system as phosphates of another base. The lime phosphate contained in urine and phosphatic calculi, even when primary, is said to be almost entirely formed within the bladder. These views, as they are not quite in accordance with commonly received clinical evidence, seem to require confirmation, but they suggest moderate expectation of cure by lime salts.

The bicarbonate, as occurring in carrara water, is soluble by virtue of the excess of carbonic acid, and readily absorbed. The neutral carbonate, in small doses (5 or 6 gr.), is soluble in the gastric juice, and is absorbed as a chloride. The chloride itself, in similar doses, and diluted sufficiently to disguise its caustic taste (as with 3 oz. of sugared water), becomes absorbed without gastric disturbance; but larger doses are apt to cause a sense of oppression, with nausea and diarrhoea. Unduly large doses of lime-water, or of phosphates or carbonates, may also cause gastrointestinal irritation.

Of that which is absorbed, an equivalent quantity is eliminated, except during the period of growth, and especially of bone-development. There seems to be a power of laying-by some of the substance for this purpose, for, e.g., during the early months of pregnancy, bony growths (osteophytes) sometimes form in the bone of the parent, which diminish with the growth of the foetus. The eliminated portion is found in the urine, as acid phosphate, and in many other secretions, such as the pancreatic juice, and the semen; some may be detected also in plastic exudations; sometimes it forms calculi. It is of ten deposited in tumors, fatty, fibrous, and sarcomatous, and in old inflammatory exudations, as in tubercle of lung and strumous glands. About 45 gr. are daily eliminated by an adult man (Husemann).

Physiological Action (External)

Lime unslaked, or "quick," decomposes and destroys organic matter, and is used sometimes as a caustic, more often as a disinfectant, e.g., in dissecting rooms and in graveyards; its affinity for water, and its ready combination with sulphur (as in sulphuretted hydrogen), will explain its good effects. It is used by tanners to remove the hair from hides, and by farmers as a fertilizing agent. Its action upon the living skin is irritant and to some extent caustic, but, as it has less "diffusion power," is more superficial and more limited than that of the alkalies proper - potash and soda. On the mucous membranes, however, its effects may be very severe, as when by accident it enters the eye, or when too strong a solution of it, or of its haloid salts, is taken into the mouth. Local inflammation and ulceration may follow, and even a fatal result be produced when the stomach is affected.

Weak solutions or the neutral salts, carbonate and phosphate, in powder, have a local astringent and sedative effect. The "lime-water" of the Pharmacopoeia is not strong enough to be caustic, but controls secretion, especially from mucous membranes, and renders any tissues pale and dry.

Physiological Action (Internal)