Inflammation of bone often arises from external injury, and in some constitutions from very slight causes. Its occurrence is supposed to be favoured by a syphilitic taint, but the inflammatory disposition is much more frequently produced in a system vitiated by the abuse of mercury. From the unyielding nature of the tissue, the pain attendant on inflammatory action is dreadfully excruciating; it is also most violent during the night, even in chronic cases, a circumstance which does not admit of satisfactory explanation. The integuments over the inflamed bone are swollen, and the tumour is œdematous; whilst a hard and solid tumefaction exists in the more deeply-seated parts, caused partly by enlargement of the osseous tissue and partly by effusion of lymph into the cellular substance. The bone is imbedded in a gelatinous or lymphatic effusion, situated mostly beneath the periosteum. This membrane is more vascular than in its natural condition, thickened, and at the same time opened out in texture. The bloodvessels of the affected bone are much increased, both in activity and in size; and, in consequence of enlargement of the vessels, and thickening of the naturally delicate membrane on which the vessels ramify, the bone is swollen and increased in size; its texture, as shown in the annexed cut, is loose, somewhat resembling the cancellated structure, and its surface is occupied by numerous foramina, which are enlarged in proportion to the size of the vessels which they contain. The limb is often enormously swollen and indurated. The gelatinous effusion beneath the periosteum speedily becomes organised, nodules of osseous matter project into it, and adhere to the surface of the bone frequently by a narrow neck; these increase in number, gradually assume a solid appearance: the bone is thus thickened, often to a very great extent. [Illustration] It has been supposed that the new osseous matter is deposited by the vessels of the soft parts and of the periosteum; but there can be little doubt but that it is secreted principally by the vessels which ramify within the substance of the bone, and by the vessels of the periosteum after they have entered the osseous tissue. Thus, in the case of fracture, the new osseous particles lie between the periosteum and bone at a distance from the broken ends, where the vessels are enlarged and increased in activity, or adhere to fragments which have been detached in part and retain their vitality, but not to the under surface of the periosteum. There is no doubt that thin laminæ of bone are now and then found attached to the periosteum, or impacted within its substance; but this is to be attributed to that morbid action of the tissue, to which this as well as several other membranes is subject.