The formation of calculous concretions in the urinary organs is a matter of extreme importance; it takes place within the kidneys, in the pelvis and calices of the kidneys, in the ureters, the bladder, the urethra, the urachus, and even externally to these passages. The pelvis and calices of the kidney and the bladder are, however, the parts in which calculi are most frequently formed. The latter present considerable varieties, both as regards their physical properties and their chemical composition.

aa. When the concretions are very small they are termed gravel, and may be very numerous or few in number. Gravel may be formed at any part of the urinary apparatus, and even in the kidney. The red variety consists chiefly of lithic acid, the white of phosphates. Calculi are larger concretions, which again differ much as to volume and weight. In size they vary from that of a millet-seed to that of a goose's egg, or a fist.

/3/3. Vesical calculi are generally of a globular, ovate, or oval form; they are frequently flattened so as to present a discoid or lenticular shape; if two or more coexist, friction planes are formed giving the calculi when numerous, a polyhedral shape. Large renal calculi are moulded according to the form of their nidus, and assume a branched appearance. In rare cases the calculi are hollow, forming tubular or conchoid concretions. Their surface is either smooth or rough, angular or fissured; or it appears decaying, gnawed, granular, of a mulberry form, or set with sharp, prickly projections, crystalline, etc.

yy. The number of the calculi present varies; there are generally several renal calculi, whereas vesical calculi are commonly solitary; however, there are cases on record in which fifty, a hundred, nay, several hundred calculi, especially of the phosphatic variety, were found.

3d. In color, consistency, and texture, they vary much, and these qualities depend upon their chemical composition.

The substances entering into the chemical constitution of urinary calculi are numerous; sometimes one only form the calculus or predominates, at others several are mixed up together, or disposed in layers. They are not all equally frequent.

a. Lithic acid enters into the composition of most calculi, inasmuch as many consist entirely of it, many in part, and as it forms the nucleus of the majority. Lithic-acid calculi are commonly of considerable hardness, smooth, light or dark brown, rounded, and often flattened.

B. Lithate of ammonia and lithate of soda rarely enter into the composition of calculi. Those consisting of the former are yellow, and of a loose texture; those composed of the latter are white and chalky.

r. Phosphate of lime rarely forms a calculus by itself.

d. Phosphate of ammonia and magnesia forms small, friable, white calculi, that have a shining crystalline investment.

Calculi consisting chiefly of the two last-named substances and carbonate of lime, are very frequent. They are white, of a loose texture, and often of a considerable size; they are generally formed in consequence of inflammatory affections of the kidneys and urinary passages, which in their turn are frequently induced by the presence of a lithic-acid calculus, or some other foreign body, which serves as a nucleus for the calculous deposit.

4. Oxalate Of Lime

Oxalate Of Lime forms the mulberry-shaped, nodulated, dark-brown or black, and very hard calculi.

C'. Xanthic oxide and cystine are very rare. The latter we generally find combined with fat, resin, coloring matter, iron, silica. In rare cases we also find fibrinous coagula, in the shape of carneous or fibrous elastic masses, entering into the formation of calculi.

Vesical calculi are either contained free and unattached in the bladder or are firmly grasped by the bladder, which has become hypertrophied in consequence of catarrhal attacks. They are found encysted in hernial diverticula of the bladder, or lie in saccular expansions of the vesical parietes, which they form for themselves during the contractions of the bladder; they sometimes become agglutinated to these and other parts by means of fibrinous exudations.

Urinary calculi offer mechanical obstacles to the conduction and discharge of the urine, and give rise to inflammations of the kidneys and urinary passages, proportionate to the size of the calculi, and the roughness and irregularity of their surface. They are sometimes, even when of considerable magnitude, discharged by the natural passages, especially in the female; still they more commonly cause severe injuries of the urinary channels, rupture of the urethra, etc. At other times they make their way by inflammation and suppuration into neighboring cavities, as into the rectum, the vagina, or into abscesses, and from these by unnatural passages outwards.

In very rare cases we find urinary calculi enclosed in cartilaginous capsules external to the urinary passages, having either forced their way out of the latter by rupture or ulcerative perforation, or having been formed at the spot where they are discovered, in urine that has been previously extravasated.