These are of very frequent occurrence, especially in people past middle life, and they are often found in the gall-bladder after death without their existence having been suspected during life.

The cause of their formation is very obscure. They are formed in the gall-bladder by the deposition of the constituents of the bile, chiefly cholestearine and next to that bile pigments, but also lime and magnesia salts in varying proportions. Probably stagnation of the bile in the gall-bladder at least predisposes to their formation, and this is rendered the more probable from their more frequent occurrence in old people where the actions are sluggish. In the centre of gall-stones there is frequently a nucleus composed of remains of epithelium or mucus, and it has been supposed that catarrh of the bladder may furnish this nucleus.

Gall-stones occur singly or in numbers. The Single gall-stones are commonly composed almost purely of cholestearine, of which there may be over 98 per cent. They are oval in shape, somewhat nodulated on the surface, and have a glistening appearance, altogether somewhat resembling in appearance a sugar plum. When divided or broken they present a characteristic radiation from the centre, and also sometimes a concentric stratification. The stone is very light in weight, floats in water when previously dried, and has a soft almost soapy feeling.

Multiple gall-stones are more frequent, and although sometimes nearly pure, they are more frequently composed of cholestearine mixed with bile pigment and lime salts. There may be two or three, but they may be present in large numbers, fifty, a hundred, or several hundreds or thousands (as many as 7800 have been counted). If few they may be comparatively large, but if numerous they are small, the bladder sometimes having the appearance of a bag filled with peas. The multiple stones are always facetted, taking their shape according as there is room, and fitting into each other. In appearance they have been compared to the macerated carpal bones, having somewhat similar facets, and often presenting a similar greyish colour, although sometimes yellow, brown, or even black. They are very light in weight, and on section present little of the glistening appearance or radiating arrangements of the single ones, being rather stratified, more deeply and less deeply pigmented strata alternating.

Gall-stones composed almost entirely of bile-pigment are very rare. They are small, nodulated, and nearly black, and occur in considerable numbers at a time. Stones composed mainly of Lime salts, especially the carbonate, are still more rare.

The condition of the Gall-bladder, which is the seat of gallstones, varies somewhat. It is often completely collapsed on the calculi, so that it is a'tightly filled bag of stones. In other cases there is a small amount of bile in the bladder, whilst in others a tough mucus surrounds the calculi.

Gall-stones frequently leave the bladder and pass into the cystic duct, where they may remain for a time and cause obstruction. After a time they often pass on into the ductus choledochus. The last part of this duct is narrow, and, if the stone is of any considerable size, it usually sticks here, at least for a time. By dilating the duct it may get into the duodenum, but very commonly it finds its way through by ulceration, and sometimes it ulcerates into the peritoneum, producing peritonitis. Lying at the mouth of the ductus communis it obstructs the outflow of bile and produces the results to be presently described. When it gets into the duodenum it usually passes off with the faeces, but if large, it may produce Obstruction of the intestine at some point. This is of rare occurrence, and can scarcely happen except in the case of large stones, chiefly of the solitary kind.

The calculus may ulcerate from the gall-bladder directly outwards. It may thus pass into the peritoneum, or into a neighbouring canal such as the portal vein. These are, however, rare occurrences.