-NON-PLASTIC PRODUCTS.
Products of this class are composed of ma terials either (first) completely inorganic ; or (secondly) of elements, though organic, inca pable of assuming organized arrangement ; or (thirdly) they are formed of a union of sub stances of both these kinds. Of the first va riety one of the most unquestionable examples is supplied by solid accumulations of calcareous salts round inorganic bodies introduced into the system from without. To the scond be long biliary calculi,— masses composed mainly of an animal substance not only unfitted to form structure, but lowered in the scale of animal existences by having a crystalline form impressed on it. Among the products belong ing to the third variety may be found, for example, certain urinary calculi, compounds of inorganic saline, and structureless animal, sub stances.
All non-plastic products agree in being di rectly derived from the fluids of the body. But they are not all developed on a uniform plan. Some of them originate in the coalescence of the more solid particles of secreted fluids, after the act of secretion is accomplished ; and this coalescence is essentially a physico chemical process, primarily, of precipitation, and, secondarily, either of crystallization or accretion. Others are exuded ready formed from the vessels. And this difference in mode of origin coincides with numerous differences in pathological relations ; hence it may be ad vantageously used in forming two sub-classes of non-plastic products ; those (Sub-Class I.) : Produced by precipitation from secreted fluids.
(Sub-Class II.) : Exuded ready formed from the vessels.