Gall stones are most frequent in animals having a gall bladder. Some medical writers say they are formed in the gall bladder only, but the soliped which has no gall bladder has in particular instances furnished hundreds of gall stones. Yet the ox, dog, sheep and pig are the common victims of biliary calculi among our domestic animals. In these the calculi appear to be mostly deposited from the stagnant bile in the gall bladder, yet concretions on the biliary ducts and hollow casts inside the ducts are by no means uncommon.
A gall stone may be single, or they may be multiple up to hundreds or even thousands, and when very numerous they are individually small, perhaps no larger than a pin’s head. They may, however, attain the size of a marble or more, and by mutual pressure and wear they assume various polygonal forms. If they lie apart in the gall ducts or bladder they are regularly rounded. They are sometimes mulberry shaped as if conglomerate. In other cases the solid masses are so small as to have secured them the name of biliary sand. Casts and incrustations in the ducts are not necessarily made up of smaller globular masses.
On section a calculus shows a nucleus, composed of bile pigment, blood, mucus, with the debris of parasites or bacteria. Around this nucleus the calculus is deposited in concentric layers, of a hard material consisting largely of cholesterine, but containing also bile coloring matter, bile salts, and lime, in short all the constituents of bile.
Causes. Various conditions contribute to the precipitation of biliary solids in the form of calculi or encrustations. The most prominent causes are: lack of exercise, overfeeding, dry feeding, concentration of the bile, the presence of colloids and bacterian infection.
Idleness is especially operative in cattle, which are quite subject to biliary calculi and concretions, when shut up in the stall on abundant, dry feeding for a long winter. They are not noticed in stalled animals, that are fed watery or succulent rations, such as green fodder, distiller’s or brewer’s swill, ensilage, brewer’s grains, mashes, roots, potatoes, apples, pumpkins, and in case a tendency to their formation is developed on the dry feeding of winter, the concretions may be re-dissolved and entirely removed by the succulent spring grass. A similar influence is noticed in the human family, as the female sex living mostly indoors, and males pursuing sedentary occupations furnish the greatest number of gall stones.
Concentration of bile results in part from muscular inactivity and hepatic torpor, but also from overfeeding which loads the portal blood and indirectly the bile with an excess of solids, and from dry feeding which lessening the secretion of water leaves the bile more dense and predisposed to precipitate its solids. The density of the liquid, however, developed from a rich and dry ration and a prolonged inactivity, may continue for a length of time, without the occurrence of actual precipitation. It usually requires some additional factor to make this predisposition a direct cause.
Presence of Colloids. This may be found in the presence of solid or semi-solid particles. Just as the introduction of a thread into a concentrated solution of sugar or salt will induce an instant crystallization on the filament so the presence of solid bodies determines a similar condensation in solid form of the solids of the bile. But this tendency is increased materially if the solid body is itself of a colloid or non-crystallizable material. Rainey and Ord have shown experimentally that colloid bodies like mucus, albumen, pus, blood, epithelial cells, not only determine the precipitation of crystallizable salts from a strong solution, but that they cause the precipitate to assume the form of globular or spherical particles, which by gradual accretions on their surfaces tend to grow into calculi. They found that salts which are deposited by mere chemical reaction, without the intervention of colloids, appear in the form of sharply defined angular crystals. The very fact that a precipitate assumes a spherical form suggests the presence of colloids as an active factor in the precipitation. Heat appears to intensify this action, though probably the normal body temperature operates mainly through the more active proliferation of bacteria.
Bacteridian infection. In connection with the action of colloids it has been observed that when such bodies are in a condition of fermentation they are much more potent as precipitants than if inactive and sterile. But as all fermentations are the work of microörganisms we are at once brought to the conclusion that bacterial infection is one of the most potent causes of calculous formations. The invading microbes operate upon the dissolved solids, causing changes in their condition which reduce their solubility, and thus determine the separation of calculi and concretions in a manner allied to the precipitation of nitrates in the soils.
But the same microbes operate in producing the colloids which coöperate so effectively in the formation of calculi. The catarrhal biliary ducts, or bladder, shed their epithelium, and transude white and red globules, and form pus and an excess of mucus, all tending to the separation of the biliary solids or forming nuclei on which these solids may condense. The calculi and concretions tend in their turn to maintain and advance the inflammation.
The access of the microbes to the biliary duct or bladder may be effected through the blood of the portal vein or hepatic artery, or in the new-born, through the umbilical vein from an infected navel. As other modes of access may be named, a gradual advance from the duodenum through the common bile duct, or more speedily on or in the bodies of parasites (ascaris, strongylus, stephanurus, tænia, echinococcus, distoma, fasciola, coccidia), etc.