This is the form in which ostitis most frequently presents itself in the horse. Ringbones, some splints, and various other excrescences on the bones of the limbs and other parts of the skeleton are frequently of this nature.

At first the affected bone becomes porous and spongy (fig. 319), as the result of the inflammatory exudation pressing upon the vascular canals of the bone and promoting their absorption and enlargement. As a result of this the bone tissue becomes changed from a close compact structure to a loose and spongy condition. This is what is known as rarefying ostitis.

As the inflammation abates, the material thrown out of the vessels into the structure of the bone, by which the rarefaction was produced, is itself converted into bone.

The effect of this is to change the part from a soft spongy condition to a state of great density and hardness (fig. 320).

Fig. 319. - Rarefying Chronic Ostitis.

Fig. 320. - Chronic Ostitis.

A, Lower portion of Radius of horse, showing results of Chronic Ostitis. B, Section of the same, showing hard, dense condition of the bone.