Tumours are divided into _Solid_ and _Encysted_. The solid are generally enveloped by a dense cellular sheath of the surrounding cellular substance, yielding and becoming condensed in proportion as the tumour increases in size; this covering appears as a barrier between the healthy and diseased parts, shutting out the latter, as much as possible, from connection with the rest of the body, and preventing the former from participating in the injurious tendencies of the latter. Some tumours have no such limit, but extend in the direction where there is the least resistance, hold a free intercourse with the surrounding parts, and impart to them their morbid disposition and action; others are limited in their situation and communications, but prove dangerous or annoying from their bulk. Some grow rapidly, and prove troublesome in a few weeks or months; others remain without much increase for years, and produce little or no inconvenience. Occasionally tumours partly resemble the texture in which they grow; those of a fatty nature are frequently found to have their nidus in the adipose tissue; cartilaginous tumours project from the surfaces of bones or of a joint, are subsequently detached, and lie loose in its cavity; growths of a cellular structure internally, and invested by an apparently mucous lining, protrude from the surface of mucous membranes. Others differ, not only from the texture in which they are situated, and from which they derive their nutritive vessels, but also from every other part of the healthy structure. In one instance, a congenital tumour was found to be composed of an aggregation of numerous materials, many of them resembling the healthy textures of the body. But again, tumours are constantly met with, composed of matter which in no respect resembles any of the natural tissues of the body: those are what have been called heterologous formations.