This disease is mostly found to exist in the long bones of the limbs of young animals when growth of the skeleton is most active, and the vessels of the membrane are highly charged with blood for the supply of its nutritive requirements.

Fig. 3I6. - Diagrammatic Transverse Section of Tibia and Fibula.

A, The two layers of the periosteum with blood-vessels, etc. B, Bone of the Tibia. c, Bone of the Fibula. D, Endosteum. E, Marrow.

It is especially frequent in the shin-bones of young race-horses, where it is commonly known among trainers and stablemen as "sore shins".

Periostitis may be acute or subacute. In the former case it soon spreads to the bone, and may cause portions to die and to slough. In the latter, which is the more common form of the disease in the horse, the action is of a more formative kind, and usually results in the thickening of the periosteum and the formation of new bone.

Two factors are concerned in this destructive process: - 1. Exudation is thrown out from the vessels within the bone into the minute canals which they traverse. This continuing, the vessels are pressed upon by the exuded matter, and the circulation of blood being thereby interfered with the nutrition of the bone suffers accordingly.

2. Exudation also takes place from the vessels beneath the periosteum, lifting the membrane away from the bone, with the result that a portion of the latter dies, partly from want of the nourishment which the vessels of the periosteum afford to it, and partly also from pressure on the vessels of the bone itself by the matter exuded around them.

Separation of the dead piece of bone from the living must in such circumstances take place. During this process abscesses form over the site of the injury, and pus (matter) is discharged by one or more openings in the skin.

Symptoms

Acute pain, great heat, and lameness are early symptoms of the disease. Pressure over the part causes sudden withdrawal of the limb. Swelling soon appears - at first firm, then less resisting, and ultimately fluctuating. An abscess forms and breaks, and finally the dead bone, if not removed by an operation, crumbles away and escapes in small particles with the pus.

Treatment

Perfect rest is the first requirement in these cases. An incision should be made through the periosteum as soon as the disease is found to exist. This will afford an opportunity for the escape of matter as it is formed, and prevent any serious separation of the membrane from the bone by its accumulation beneath it. Where this has already taken place it is desirable to make a bold opening, and after irrigating the wound freely with antiseptic solution continue the treatment on the principles laid down for dealing with wounds on the antiseptic system. It should not be overlooked to remove any dead piece of bone that may exist, as soon as it is sufficiently detached to be taken away.

Acute periostitis is only of seldom occurrence, and perhaps the most common examples are those which occur in the lower jaw as the result of injury inflicted by the bit. Occasionally it is seen in the bones of the extremities after severe blows.