Compound fractures unite in a different manner from that by first intention: yet the repair by suppurative inflammation, in its essential particulars, has been far too little investigated. It is analogous on the whole to the repair by first intention, certain stages of the latter always occurring in the course of it; only, as must be obvious, the repair is effected with more difficulty and at a later period, and sometimes is never completed.

In this as in the other process, a capsule of soft parts forms around the broken ends of the bone, but the inflammatory product contained within it is pus, just as in any wound which may exist at the same time in the soft parts, and the capsule being lined with a grayish-red, trans-lucid, jelly-like, granulating layer, which is covered with pus, is in fact a closed abscess.

The early callus, as in union by first intention, appears as a gelatinous exudation, which subsequently becomes a cartilaginous, and then a bony stratum. It springs from the bone, at the part which has remained covered by periosteum; and then, keeping off from the denuded ends of the bone and from the fracture itself, it advances on the inner wall of the capsule towards the broken part, the ossifying cartilage being developed from the granulating layer.

The broken ends of the bone, so far as they are denuded of their periosteum, are washed in the pus: they lose their natural color, and look bleached and dull white, but are otherwise unchanged - even at the surfaces and margins of the fracture.

It is not usual to see any plastic exudation in the open medullary cavity: the fractured surfaces are everywhere washed in the pus: the internal surface of the bone - that which faces the medullary cavity - appears dull white and dead to a greater or less depth on one side, or for the greater part, or the whole, of its circumference: the cells and spongy tissue are in the same state; and the medulla at that part is collapsed, soft, and discolored, and is dissolving in the pus. Beyond the confines of this change, however, the medulla is swollen and reddened, and if the necrosis have not extended all round the bone, it protrudes, as it does in the process of union by first intention, beyond the fractured surfaces.

Now, it is more particularly the growth of secondary callus which is late in commencing, which very often suffers more or less considerable interruptions in its progress, and which differs most from that which is formed in union by first intention. The ends of the bone being washed in pus, and thus kept in contact with a fluid incapable of organization, die: and the extent of the necrosis, whether it shall be superficial, shall be confined to the inner layer of the bone, or involve its whole thickness, is unquestionably determined by the extent to which the periosteum is stripped off, and the medullary membrane destroyed. As soon as the necrosed part has exfoliated, granulations appear on the bare surfaces and margins of the fragments, and become the basement in which the new bony substance, or secondary callus, is deposited.

It is by the very slow manner in which the exfoliation of the necrosed bone takes place, that the formation of the secondary callus is delayed: and as not unfrequently the powers of the system are almost exhausted before it is completely formed, it is often produced in insufficient quantity, or arrested in its perfect internal development, that is, in its ossification. Moreover, as soon as the exfoliation and entire removal of the necrosed piece are effected, the inflammation ceases, and a great part of the granulations which were exuded for the purpose of producing new bone, goes to form on all sides a cellulo-fibrous, ligamentous, and cicatrix tissue: hence it is, that the repair of fractures in this manner is so often incomplete, and is attended with so extensive and permanent a loss of substance. Whatever the condition of the early callus, whether it be abundant in quantity and thoroughly organized, or otherwise, the granulations supply an inadequate substitute for the bony substance which has been lost by exfoliation: instead of changing into cartilage and bone, they become converted into fibroid tissue; and thus the repair which ensues is attended with shortening, with disfiguring cicatrices, or with an artificial joint.