A caries[36] of the bone is at all times a disease which is difficult to manage, both on account of the mechanical obstacles which we have to overcome, and the slowness with which the affected parts perform their actions either of disease or recovery. The divisions of this disease have generally been taken from the appearance of the caries, and its extent. We have the dry caries, the worm-eaten caries, the spongy, or carnous caries, &c. and we have the deep and superficial. But, as the cure of these is to be conducted on the same principles in all of them, and as they are most probably different degrees of the same complaint, it will be more useful to divide them into those which affect bones lying deeply, and those which affect the more superficial bones; because these different cases are attended with very different circumstances and symptoms. The first is preceded by an abscess, which forms generally with much pain, runs its course slowly, and does not burst for a considerable time. When it does open, its sides do not ulcerate, at least universally, but a sinus remains, the mouth, or exposed part of which only, assumes the ulcerative action. The second is more rarely preceded by any abscess, but is either coeval with the ulcer of soft parts, (both being produced by mechanical violence) or it succeeds the ulcer, and is caused by it. This ulcer belongs to the third genus, and will be afterwards described. [36] When a bone becomes carious, the periosteum is completely detached, and, therefore, it is felt to be rough by the probe. Its colour becomes first of a dull white, or dirty yellow, which it either preserves, or changes for the intermediate hues betwixt these and black. It is generally more porous than formerly, and lighter; but these qualities vary, from very slight degrees, to the appearance of a light coralline.