When the extravasation produced by the fracture of the bone and simultaneous injury of the soft parts, and the vascularity of the soft parts, of the lacerated periosteum, the surrounding cellular tissue, and the adjoining muscles and their sheaths, have in some degree subsided, a reactionary process of inflammation is set up. The soft parts around the fracture, some of which have been injured by the same violence that produced the fracture, and others by the broken bone itself, become swollen and so blended as to constitute one uniform red, firm mass, infiltrated with inflammatory product, which encloses the broken part in a more or less smooth and round, or in an elongated, swelling. In the same manner, the medullary membrane becomes tumid and red; and, after a time, puts forth a red loose mass, which clings to the broken surfaces of the bone, but soon coalesces with the surrounding soft parts (Breschet's substantia intermedia). Thus each of the fractured ends lies in a sort of capsule of swollen soft parts. The innermost layer of the capsule is the periosteum, which, having separated from the surface of the bone to a greater or less distance from the fracture, is so intimately united with the surrounding soft parts as to be no longer distinguishable: a viscous reddish fluid exuding on its inner surface, fills up the space between it and the ends of the fragments.

Meanwhile, commencing at different distances from the seat of fracture, along the line where the periosteum remains connected with the bone, a reddish, semi-fluid, gelatinous substance exudes, which is greater in quantity the nearer the fracture: it is evidently connected more intimately with the bone than with the periosteum, but the former is entirely unaltered beneath it. Where it adjoins the bone it become cartilaginous, and subsequently ossifies; it then adheres intimately to the osseous surface, and if stripped off, leaves it perceptibly rough, and with its pores dilated.

This substance, as it increases in quantity, advances from the point at which the bone and periosteum are connected towards the seat of fracture, keeping close to the inner surface of the capsule of soft parts, and leaving a space between itself and the bare extremities of the bone, which is filled with the reddish viscid fluid before mentioned. At the same time, as it ossifies, it gradually assumes a more definite internal structure. If the fragments of the bone be in favorable position, the masses of callus, as they grow, assume a cylindrical shape, and arrive at the septum formed by the substantia intermedia. The septum has already acquired a cellulo-fibrous texture; but now it is gradually removed by absorption, and the masses of callus from the opposite ends of the bone coalesce over the fractured spot. What takes place outside the bone goes on also in the medullary cavity; a substance is effused, which everywhere ossifies and obliterates the cavity of the medulla. These changes constitute what is called the formation of early callus.

This callus gives some firmness to the fracture, and the time occupied in completing it - from thirty to forty days - forms, in a practical point of view, a very important period in the whole reparative process. It is named by Dupuytren the provisional callus.

Long before it has reached the state of completeness above described, a later growth, or formation of definitive callus, commences.

The fluid before spoken of as occupying the space between the broken ends of the bone and the callus (and a similar fluid which may be found also between the callus and the soft parts) becomes gradually more firm, receives vessels, and acquires a structure which at first resembles granulations, but at a later period is cellulo-fibrous. A small quantity of a reddish exudation appears beneath it on the denuded ends of the bone, and gradually unites with the granulation-like substance.

As a vascular communication is thus established between the surface of the bone and the surrounding soft parts, a formation of new osseous substance commences all round the fractured ends: it resembles the first formation of callus, but proceeds with less energy.

Both the earlier and the later callus increase in quantity, and at length unite together. They are, however, distinguished from each other by the difference of their texture, particularly of the softness of the second callus, when compared with the fully formed earlier growth. In this manner the broken extremities of the bone are surrounded by an uninterrupted osseous sheath which adheres to them both.

Last of all, in four or five months or more after the injury, the broken surfaces themselves unite within this sheath, though, indeed, the first traces of exuding osseous substance are sometimes perceptible much earlier, beneath the substantia intermedia, where it clings to the margins of the fracture. The substantia intermedia then disappears, and the fracture is completely repaired. The edges of the fracture are pretty frequently, though not always, distinctly rounded off.

During this consolidation, the remaining swelling of the soft parts subsides, and the medullary cavity begins to be restored at the fractured spot. The Haversian canals, in the mass of bone with which the cavity is filled, are gradually enlarged to such an extent as to render it cellular and areolar instead of solid; by further absorption it is entirely removed, and a new medullary cavity occupies its place. Sometimes a thin layer of the mass remains behind, united at the seat of fracture with the bony substance exuded from the broken margins, and for a long time obstructs the canal.

The callus, which was more or less uneven externally, has by this time become smooth. It is invested with a fibro-cellular membrane, which consists partly of the old periosteum of the bone thickened, and partly of the exudation poured out by the soft parts: the latter forms a new periosteum, unites into one membrane with the old one, and gradually becoming thinner, at last precisely resembles healthy periosteum.

Although the callus even when completely ossified, is at first clearly distinguishable from the old bone by the arrangement of its canals and lamellae, yet in the end it acquires precisely the same characters as the bone. It is more than probable, that in process of time the callus diminishes in bulk, very gradually indeed, but yet so far as the relative position of the fragments of the bone, and its own original size, allow. Thus where simple transverse fractures have united, it shrinks so much as to form at last an inconsiderable elevation, which points out the original seat of fracture. But this is by no means to be regarded as an atrophy of the so-called provisional callus: no such decay, or involution, ever takes place, though it has been asserted by Dupuytren, and admitted by several others after him: there is no provisional callus, in the sense of a material for temporarily uniting or soldering the fragments together; and the reabsorption of the mass of bone which fills the medullary canal near a fracture, is to be regarded as one of the phenomena in the progress of the formation of a bone.

The firmness of the union is generally proportioned to the length of time that has elapsed since the occurrence of the fracture.

The process just described, in which bones reunite by first intention, is on the whole the same as that which takes place in injured soft parts. I have now to refer briefly to some points connected with the process, which have long been subjects of dispute; to describe what is observed when fractures unite under other, and especially under less favorable conditions than those which we have supposed in the foregoing delineation; and lastly, to treat of more important anomalies in the process, such as arrest, and the formation of new or false joints, repair by way of suppuration, and necrosis.

Long as the contrary opinion has been entertained, it is now beyond doubt, that in the formation of callus, no ossification of the periosteum takes place, any more than of the surrounding soft parts; but that, in the first formation of callus, the development of the whole osseous mass proceeds from the bone only is not fully ascertained, probable though it be. For, not to mention other facts, bone is sometimes found on the inner surface of periosteum; and the dura mater particularly, which is virtually a periosteum, frequently presents bony growths on its outer surface, which do not adhere to the bone at all, or do so very loosely, and only at a later period become closely connected with it.

During the whole process of forming callus, the old bone undergoes very trifling change; and it requires close examination after the removal of the exudation, to discover that the pores on its surface are somewhat enlarged, and that the surface itself is rather rough, in consequence of some of the new matter remaining adherent to it. There is no greater change even when the exudation has ossified. When the formation of callus is complete, and the fragments have reunited, the bone sometimes shrinks, and becomes palpably thinner, and its cavity smaller than before.

When only one of two adjoining bones is broken, as in the forearm or leg, the uninjured one takes part in the process of forming callus. Osseous matter is exuded by it near the fracture, which unites with the callus of the broken bone; for just as the substantia intermedia disappears when the broken surfaces of the bone unite, so the periosteum covering the callus of the uninjured bone is absorbed, and the two growths of callus coalesce. But still more frequently is this the case when both bones of the forearm or leg are broken, or whenever there is a fracture of two or more adjoining bones, such as the ribs. In fractures in the neighborhood of joints, fears may even be entertained, lest union between the articular ends of the bones be occasioned by the callus.

Long oblique fractures involve a mass of soft parts in the reactionary process, and are attended by the formation of a quantity of callus, proportioned to the extent of the fractured surfaces. So, too, the greater the displacement of the fragments, the greater and at the same time the more misshapen, will be the swelling of the soft parts, and the subsequent callus. The same occurs when the fracture is comminuted.

The great extent to which fragments, when displaced, are denuded occasions a difficulty in the formation of the secondary callus, and retards the process of repair; but in all essential particulars the process is the same. The callus encloses the fragments, and generally forms a bulky mass, which connects the opposite surfaces of the bone together. The medullary cavities, at first open, become blocked up with callus, which unites intimately with the swollen soft parts around; but gradually opening again, they become continuous with a new medullary cavity, which forms in the callus. Sometimes even when contiguous walls of the bone are firmly consolidated together, they and the intervening callus are absorbed, and the continuity of the original tube is restored by means of a new transverse or oblique canal. A remarkable analogy to the mode in which bone is originally developed is observed in the universal tendency to form medullary cavities in the interior of all large masses of callus, as in exostosis, and the more bulky osteophytes.

Separate fragments unite within the callous enlargement in more or less favorable positions.

The surface of large masses of callus is marked with grooves in which tendons or large vessels lie; and sometimes they are even perforated in various directions by small canals which those structures traverse.