This word, the strict meaning of which is only mortification, is by general consent confined to this affection of the bones It was first used in this particular sense by the old writers, who restricted its application, however, to examples in which the whole thickness of a bone was destroyed. By the ancients, the death of parts of bones was not distinguished from caries; but necrosis and caries are essentially different, for in the first the affected part of the bone is deprived of the vital principle, but this is not the case when it is simply carious. Caries, as I have already stated, is very analogous to ulceration, while necrosis closely resembles mortification of the soft parts.

Between caries and necrosis there is all that difference which exists between ulcers and gangrene, or sphacelus of the soft parts. In caries the nutrition of the hone is only impaired, and an irregular action disunites the elements of the bony structure, which consequently sustain a loss of substance; but every remaining part of it is yet alive. In necrosis, on the contrary, the vitality and nutritive functions cease altogether in a certain portion of the bone, the separation of which then becomes indispensable. It therefore follows that a true necrosis must always he said to exist whenever a dead portion of bone has either separated, or is about to separate. Lastly, necrosis may affect the long hones, or the broad, the large, or the small, and even those of the very least size.

Besides the differences arising from the particular bones affected, necrosis also varies, according as the portion of hone attacked happens to be thin and of little extent, or large and of considerable thickness. The disease is simple when it is confined to one bone, compound when several parts of the same bone or several distinct bones are affected at the same time, and when other parts of the body are also diseased. It should also be known, because the information is of practical importance in the treatment, that necrosis has three different stages or periods. In the first the bone affected perishes; in the second, the process of exfoliation or separation of the dead bone from the living is going on; and in the third, the separation is completed. The causes of necrosis are not essentially different from those which produce ulcers and gangrene of the soft parts. As, however, the vitality of the bones is weaker, we may infer that necrosis may be occasioned in them by causes which are less numerous and intense, and as such would only give rise to suppuration in the soft parts.

Everything, whether in the periosteum (that is, the covering of bone) or the substance of the bone itself, that tends to interrupt the nutrition of the bone, must be regarded as conducive to the origin of necrosis. It is observed, however, that when the mischief in the covering or substance of the bone is of a trivial extent the consequences may be merely an abscess.