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If we suppose that the fighting was slower and less fierce in the Bovidae, so that the skin over the exostosis was subject to friction but not lacerated, the result would be a thickening of the horny layer of the epidermis as we find it, and the fact that the skin and periosteum are not destroyed explains why the horns are not shed but permanent.

In the vertebræ, operative interference is usually restricted to opening and draining the associated abscess. #Nature's Effort at Repair.# In cases which are left to nature, and in which necrosis of bone has occurred, those portions of the periosteum and marrow which have retained their vitality resume their osteogenetic functions, often to an exaggerated degree.

The periosteum is abnormally thick and vascular, the superficial layers of the bone become injected and porous, and the bones, as a whole, are thickened. When the whole thickness of the soft tissues is destroyed by the ulcerative process, the area of bone that comes to form the base of the ulcer projects as a flat, porous node, which in its turn may be eroded.

Structurally, two main groups may be differentiated: the soft, rapidly growing cellular tumours, and those containing fully formed fibrous tissue, cartilage, or bone. The soft cellular tumours are composed mainly of spindle or round cells; they grow from the marrow of the spongy ends or from the periosteum of the long bones, the diploë of the skull, the pelvis, vertebræ, and jaws.

How may the per cent of animal and of mineral matter in a bone be determined? What properties are given the bones by the animal matter? What by the mineral matter? Locate the bone cells. What is their special function? State the plan by which nourishment is supplied to the bone cells in different parts of the bone. Give the uses of the periosteum. State the purpose of the Haversian canals.

The infant cries when the part is touched; and as it does not move the limb voluntarily, the condition is spoken of as the pseudo-paralysis of syphilis. Recovery takes place under anti-syphilitic treatment and immobilisation of the limb. Diffuse thickening of the shafts of the long bones, due to a deposit of new bone by the periosteum, is sometimes met with.

It may originate in the periosteum, or may spread thence from the marrow, or from synovial membrane.

The separated epiphysis may be kept in place by the periosteum, but when this has been detached by the formation of pus beneath it, the epiphysis is liable to be displaced by muscular action or by some movement of the limb, or it is the diaphysis that is displaced, for example, the lower end of the diaphysis of the femur may be projected into the popliteal space.

Treatment. If an abscess is suspected, there should be no hesitation in exploring the interior of the bone. It is exposed by a suitable incision; the periosteum is reflected and the bone is opened up by a trephine or chisel, and the presence of an abscess may be at once indicated by the escape of pus.

The periosteum is more easily detached, is thicker than normal, and is actively engaged in forming bone. [Illustration: FIG.