Necrosis in bone corresponds to mortification in the soft parts, more particularly to dry gangrene, or mummification. It has in general a less serious character than the latter, inasmuch as by the application of appropriate artificial remedies, it very commonly, though slowly, gets well; and is only fatal in the few cases in which the strength of the patient is exhausted by the excessive secretion of matter that takes place, for the purpose of removing the dead piece of bone. Necrosis sometimes arises from external causes, such as injuries of various kinds, by which the bone is shaken, crushed, or laid bare,from the influence of severe cold, heat, etc.; and sometimes it is developed, as is said, from internal causes, amongst which various kinds of constitutional disorder are enumerated, especially scrofula, syphilis, the state of constitution resulting from the abuse of mercury, gout, scurvy, etc, and the cachectic state succeeding acute exanthemata, especially variola and measles. An internal cause of this kind, in producing necrosis, may do so, not only by exciting inflammation and suppuration of the periosteum or medullary membrane, but also, as is extremely probable from analogy, by leading to inflammation of the bone.

Necrosis may affect either the whole of a bone (necrosis totalis), or, which is more frequent, only a part of one (necrosis partialis). In the latter case the outer lamella may be alone involved (necrosis externa superficialis), or only the inner layer of a cylindrical bone, or diploetic substance of a broad bone (necrosis interna, centralis); or again the whole thickness of a bone, within certain limits, may be necrosed. But it is very rarely that a necrosed piece admits of its whole extent being so accurately defined; that which at one part includes the whole thickness of a bone, runs out at its extremity into a superficial necrosis. Internal necrosis, too, occupies a bone very unequally, and at some parts frequently extends nearly through its whole thickness.

Moreover, the boundaries of a piece of dead bone are irregular in every direction; its margins are notched and sinuous; and its thickness, especially if the piece be peripheral or central, is very unequal in different parts.

The less vascular compact bones are those which are chiefly liable to necrosis; and of such the shafts of the long bones, more particularly the tibia, and after it the femur, humerus, ulna, radius, and fibula, and the bones of the skull, are most affected. Unlike caries, it rarely occurs in spongy tissue. Necrosis of the long bones very commonly terminates at their cancellous articular extremity, or at the junction of the epiphysis with the shaft. Every period of life is subject to it, though it is most frequent in young persons; and in them it occurs as scrofulous necrosis, or necrosis based upon a scrofulous constitution.

Necrosis sets up an active inflammation in the adjoining healthy bone and surrounding soft parts, which goes on to suppuration, and continues until the dead piece is removed either by nature or art. Separation in the former manner is very seldom completed, for the suppuration rather exhausts and destroys the patient. The matter discharges itself externally by one or, more commonly, by several ulcerated sinuses.

The purpose designed in the suppurative inflammation is to separate, and finally to throw off, the dead portion of bone, which then obtains the name of sequestrum. The necrosed piece is at first distinguishable, to a certain extent, from healthy bone by its bleached and somewhat discolored appearance, but its boundaries are at that time indistinct, inasmuch as the discoloration of the dead part blends gradually with the color of the healthy bone..

All around the necrosed portion, that is to say, at its margins, and at the part where its surface is opposed to that of the healthy bone, the latter undergoes a gradual expansion or rarefaction of its tissue by the enlargement of its Haversian canals, assumes a rosy color, and becomes succulent. It acquires gradually an areolar structure, and is thus more rarefied: at length it disappears altogether, and a red soft spongy substance, a layer of granulations, occupies its place. This change is produced by an inflammatory process, which gives rise to suppuration and granulation: the bony tissue, beginning with the Haversian canals, is dissolved by the matter secreted within them, while the granulations which shoot forth at the same time, fill up the enlarged canals. The immediate result of this process is the formation of a furrow of demarcation which encircles the margin of the dead bone, and is filled with granulations; and so far as the process is completed on that surface also of the living bone which faces the dead, so far is the sequestrum separated. In this process, that is, in the solution of the layer of healthy bone adjoining the dead by means of the purulent matter, and in the attendant formation of granulations, I find enough to account for the demarcation and separation of the sequestrum; and the absorption which has been assumed to go on at the borders of a portion of necrosed bone I hold to be incompatible with the inflammatory process, while the analogy of the process by which mortified soft parts are cast off renders such a view inadmissible.

The granulations not unfrequently perforate the sequestrum where parts of it are thinner than the rest; and if this should occur at several spots, the dead bone may be completely covered by them. They have the appearance of being developed from the bone, and they fix it so as to delay its removal. It cannot be admitted, even in this case, that the sequestrum undergoes any absorption, but its perforation may be effected by the solving or corrosive power exerted on its tissue by the matter; and this further fact may be attributed to the same power, that, independently of those irregularities on the sequestrum which arise from the unequal thickness of the bone that has perished, that side of it which faces the suppurating tissue appears rough, worm-eaten, discolored, and black. There can be little doubt, indeed, that a sequestrum might be removed in this manner altogether; although at present we are without any observations on the subject made with sufficient care and accuracy to establish it as a fact.

While this process is going on, the dead bone is being replaced by a process of regeneration, which I proceed to describe as it takes place in the different forms of partial necrosis.