Frequently a bone is much enlarged throughout its whole extent, or the greater part of it, and presents a cancellated texture: sometimes, also, it is much thickened, and, at the same time, of great solidity; but such enlargements cannot be considered as tumours of bones, or exostoses, any more than those nodules of new osseous matter, which are effused in consequence of inflammation of the osseous tissue. The most frequent cause of exostoses appears to be external injury; their progress is slow, attended with slight dull pain, and often accompanied with no inconvenience; their existence can be readily ascertained, a hard and immoveable body being felt where no bone exists in the natural state of parts; but when the tumour projects into an internal cavity, the diagnosis is rendered obscure. Most frequently, they remain stationary, after having attained a certain size, and are productive of little inconvenience, the surrounding parts having accommodated themselves to the new formation. Occasionally, suppuration occurs in the soft parts, the matter comes to the surface, and a troublesome abscess is formed. To this class of tumours would I confine the term exostosis, not including those consisting of softer materials, and possessed of a less benign action. OF OSTEOSARCOMA. [Illustration] By this term is meant, an enlargement and alteration in the structure of a bone, accompanied with the deposition of a morbid sarcomatous substance internally. This morbid change appears to be the consequence of inflammation, and its origin is frequently attributed to some mechanical injury or local irritation. In the commencement of the disease, the bone is slightly enlarged, perhaps somewhat thickened in its outer laminæ; and on a section of it being made, is found to contain a brown fleshy substance instead of its cancelli. This appears to be formed in consequence of a morbid action, perhaps inflammatory, of its internal structure. By the pressure of the new formation, the parietes of the bone are pushed outwards, in some cases attenuated, in others thickened by deposition of new osseous matter, inflammatory action having been induced by the pressure. As the internal formation increases, the parietes are extended, and are generally much attenuated, becoming in some places thin as paper, and diaphanous; they also would seem to lose a portion of their earthy matter, for they are flexible, somewhat elastic, and not of their usual density. Frequently they are in several places deficient, and their situation occupied by a membranous expansion, sometimes thin and delicate, but mostly thick and ligamentous; in some cases, the external lamina appears to be converted into a substance resembling the internal growth, with which it is continuous. The investing periosteum is much thickened, and its bloodvessels are enlarged. Occasionally, the deficiency of the bone is not supplied by any membranous expansion, and the morbid growth protrudes, fungous.