Tuberculosis of bone always assumes the phase of miliary lesions, followed by the formation of a granuloma, which may gradually encroach upon surrounding tissues or may assume a more fulminating type and spread rapidly. Apparently because of the circulatory conditions these lesions generally occur near the epiphyseal lines of the long bones, apparently seeking the ends of the bones, as pulmonary lesions seek the terminations of the lungs. These lesions may be solitary or multiple. Beginning always minutely, they spread so as to produce foci perhaps two inches in diameter. As the result of the formation of granulation tissue, the surrounding bone melts away and disappears, the result being a great weakening of its structure and expansion of its dimensions in order to make room for the growing mass within. The tendency of this granulation tissue thus imprisoned is always to escape in the direction of least resistance. This carries it sometimes into the joint, sometimes out through epiphyseal junctions, and sometimes through channels in the bone made by its own pressure, with external escape and appearance of the dusky distinctive tissue, felt beneath and then upon the skin. Where bone is so weakened in one direction it is usually strengthened by compensatory deposition of calcium salts at other points, and the result frequently is a striking _combination of osteoporosis_ in the immediate presence of the disease, with _osteosclerosis_, sometimes to a remarkable degree, even to _eburnation_, of an adjoining portion. When this mass undergoes caseous degeneration the progress of the disease is much slower and the pain less. When it undergoes suppuration there are more evidences of inflammation, with more pain and systemic disturbance, as well as local swelling, tenderness, etc. The surrounding musculature is rarely involved, although the periosteum is nearly always so. In fact, it is stated that in an inflamed and suppurating bone lesion, if the muscles are extensively invaded, it may be regarded as of syphilitic rather than of tuberculous origin. The _pyophylactic_ membrane already alluded to is seen in almost every instance of tuberculous disease. The spina ventosa of some writers refers to the expansion of the shaft and medullary cavity of a long bone whose interior is occupied by a mass of tuberculous gumma, which is perforated at one point, and through which opening it escapes as does lava from a crater, to involve the structures on the outer side. The appearance of this granulation tissue in joints as _fungous tissue_ has already been mentioned. In a general way it preserves its fungoid characteristics until attacked by pyogenic or saprogenic organisms, when it quickly breaks down, forming an ulcer if upon the surface, or a cold abscess if not externally open. Tuberculous disease of the bone is most common in the young, and in them the majority of tuberculous joints are those whose bony structures have been first involved.