Other processes cause a gradual molecular softening and destruction of bone, which ultimately may be very extensive, but at no time is there present any appreciable large mass of bone. Destruction of this sort is described by the clinical term _caries_. As a means of differentiating clinical conditions, the use of these two words is desirable. As a clinical term, _necrosis_ usually means destruction by pyogenic infection, and _caries_, destruction by the gradual extension of a tuberculous process. This clinical distinction, however, is not an exact one, because destruction of large areas of bone, described as necrosis, is occasionally brought about by syphilitic infection, and rarely by tuberculosis, whilst molecular destruction of the bone is brought about by a considerable variety of processes, the chief of which, it is true, is tuberculous infection, but actinomycosis and syphilis may both lead to the gradual disintegration of the bone, without the formation of large necrotic masses of bone. The presence of necrotic bone connected with the surface of sinuses, from which comes a discharge of pus, should always lead to the consideration of tuberculosis, actinomycosis, and syphilis. The presence of large sequestra of bone should immediately suggest the presence of osteomyelitis or of syphilis. +Treatment.+The details of the treatment of the various forms of destructive processes in bone will be found under their special headings, chiefly under osteomyelitis and tuberculosis. In all cases of caries it is desirable to remove completely the softened areas in the bone. This may be done by curettment and drainage, or by excision of the entire bone, or series of bones, in certain cases, or rarely by amputation. The difficulty in all these cases is to recognize the exact limits of the carious process. It must be borne in mind that at the time of operation upon carious bones the field of vision of the surgeon is almost always limited; moreover, the bleeding which always takes place from the bone-marrow in such cases, also obscures the field, and even if these two causes were not present, it is frequently extremely difficult, by naked-eye examination to determine the exact limits of the destructive process. As a general rule, it can be said that the carious area is at least a quarter of an inch wider than appears upon visual inspection. In cases of necrosis with large bone defects, the difficult thing is to cause a growth of the bone toward the central cavity after removal of the sequestrum. The various methods applicable to such cavities are mentioned in detail under “Osteomyelitis.” +PERIOSTITIS+