In the case of simple fractures or of those which are protected from septic contamination, the process of healing generally begins soon after the infliction of the injury.

According to Ogston, in compound fractures and dislocations, the forcible dispersion of fluids by the agent producing the injury, as well as the dragging on tissues of varying degrees of distensibility, often leads to the formation of extensive spaces or cavities beneath the skin, amongst the muscles and elsewhere. As these may be the seat of septic processes they require careful attention.

The actual injury induces an acute inflammation. There is hyperemia, serous exudation and exudation of leucocytes, and there is also a certain amount of haemorrhage. These inflammatory manifestations are present in the torn periosteum and in the medulla. The acute inflammation, however, usually subsides in a few days; its prolongation, as in septic cases, is inimical to healing. Any considerable accumulation of blood is also inimical to healing, but as a general rule the effused blood, having been transuded into the tissues around, passes into the lymphatics, or, if it remain more permanently, it is disposed of by being eaten into by the new-formed inflammatory tissue.

On the subsidence of the acute inflammation the soft structures show active changes. By a process of karyomitosis (as observed by Kraft) the subperiosteal and medullary cells proliferate, so as to form granulation tissue. A similar process is visible in the endothelium of the vessels, by which means new vessels are formed. The tissue of the bone itself partakes in the process, so that it is opened out by a rarefying ostitis. We have thus a vascular granulation tissue formed in these three situations, but its amount varies greatly according to circumstances.

Fig. 275. - Green-stick fracture of femur of infant. The septum or internal callus consists of bone and cartilage posteriorly, and of fibrous tissue anteriorly.

From this tissue the Callus is formed. This name is given to the new-formed tissue which in the first instance unites the broken ends of the bones. It may be of considerable bulk as shown in Fig. 276, so as to form a kind oT case for the fractured ends; it may be very scanty in amount as in the case of incomplete or "green stick " fractures (Fig. 275). The callus, as shown in Fig. 277, consists under varying circumstances of Different forms of connective tissue. The formative cells produced in the way mentioned above have the characters variously of osteoblasts, fibroblasts, or chondroblasts, and may develop bone (c, c, c in figure), cartilage (b, b, b), or connective tissue (d, d, d). The form of tissue thus produced seems to depend on the condition of the parts as to movement. In parts where there is little movement bone will form, where there is much movement fibrous tissue, while cartilage takes an intermediate position. From this it arises that in man, where the bones are usually kept rigid by splints, the callus is formed chiefly of bone, whereas in animals it is largely formed of cartilage and fibrous tissue. But in man, where circumstances do not allow of the parts being kept at rest, then we have cartilage and fibrous tissue along with bone. This was the case in the specimen from which Figs. 276 and 277 were taken. A number of fractures had occurred in the ribs in an insane person, and they were only discovered after death.

Fig. 270. - Fracture of rib with callus, seen in section. The broken ends are shown overlapping each other. These are enclosed in a sheath of callus. Natural size.

It not infrequently happens that a Piece of bone is detached and isolated. This, however, does not interfere with the process of healing. If small, the piece of bone will survive, and having acquired vascular adhesions, will undergo a rarefying ostitis and assist in the production of callus. If large, or if unfavourably situated, it may die, and in that case it undergoes absorption by the granulation tissue (see under Necrosis). In compound fractures it is not uncommon to see a piece of dead bone eaten into and penetrated by granulations.

Fig. 277. - Longitudinal section of a fractured rib, showing callus, etc.; the section is imperfect at the upper part, a, a, a, a, a, the broken extremities of the rib dove-tailing into each other: b, b, b, new-formed cartilage constituting part of the callus; c, c, c, new-formed bone constituting the external layers of the callus; d, d, d, d, new-formed connective tissue constituting the more immediate bond of union between the broken ends, x 11.

The callus produced as above is generally described as Provisional callus, because much of it is removed before complete restoration. Names are also given according to the position of the callus. External callus is formed at the surface from the subperiosteal layer, Internal callus is formed from the bone-marrow and endosteum (Fig. 277), while Intermediary callus is between the ends of the bones and produced, to some extent at least, from the bone itself.

When the bones are fully united any excess of callus is disposed of, and little is left but what is necessary to fill up the space between the fragments as seen in Fig. 278. There also occurs a process of moulding by which the architecture is restored according to the principles already mentioned. When the bone comes again into active use there is an absorption and new-formation so as to enable the bone to resist the pressure and traction to which it is liable. Where the bones are united considerably out of position as in Fig. 279, the internal architecture is greatly modified.

Fig. 278. - Healed fracture of skull. A severe injury was inflicted years before. The parts had become completely consolidated and the edges rounded, but with the displacements shown.

From what has gone before it appears that the amount of actual bone entering into the callus varies greatly, and there may be more fibrous tissue ultimately than bone. This occurs especially when the parts are not kept at rest. On the other hand the whole process may be a very inactive one and from tearing of vessels or otherwise the blood-supply may be deficient. In such ways as these arise Ununited fractures, in which the bones are connected with fibrous tissue. In some cases there is even a kind of False joint or Pseudarthrosis formed, with smooth articulating surfaces and an approach to a synovial cavity.