Anatomy and Function of the Cartilages.—­Surmounting each wing of the distal phalanx (os pedis) is the irregularly-quadrangular cartilage.  The superior border of this cartilage is thin, generally convex, and perforated for vessels to pass to the frog; the inferior border is attached to the wing of the third phalanx and posteriorly, it is reflected inward and is continuous with the inferior surface of the sensitive frog.  The anterior border which is directed obliquely downward and backward becomes blended with the anterior lateral ligament of the coffin joint.  The fibrous expansion of the anterior digital extensor (extensor pedis) is united to the anterior borders of the lateral cartilages.

According to Smith[27]:  These structures form an elastic wall to the sensitive foot, and attachment to the vascular laminae; they also admit of increase in width occurring at the posterior part of the foot without destroying the union of the two set of leaves.  Further, by their connection with the vascular system of the foot, their elastic movements materially assist the circulation.  The primary use of the lateral cartilages is to render the internal foot elastic, and admit of its change in shape which occurs under the influence of the weight of the body.  The alteration in the shape of the foot is brought about by pressure on the pad, which widens and in consequence presses on the bars.  The pressure received by the pad is also transmitted to the plantar cushion, which likewise flattens and spreads under pressure.  Both of these factors force the cartilages slightly outwards.  When the posterior wall recoils the cartilages are carried back to their original position.  Should the elastic cartilage under pathological conditions become converted into bone, its functions are destroyed, and lameness may occur.

Etiology and Occurrence.—­The causes of ossification of these cartilages are several.  No doubt there exists a predisposition to this condition for it is of such frequent occurrence in heavy draft types of horses.  Concussion plays an important role and, according to Moeller’s[28] theory, which is sound, high heel calks prevent the frog from contacting the ground, and as weight is placed upon the foot “the lateral cartilages are subjected to a continuous inward and downward dragging strain.”

[Illustration:  Fig. 31—­Ringbone and sidebone.]

The condition affects the cartilages of the fore feet more frequently than those of the hind and the outer cartilage is more often ossified than is the inner.  This fact may be accounted for by its more exposed position; it is also frequently injured by being trampled upon and otherwise contused or cut, as in lacerated wounds of the quarter.

Symptomatology.—­Ossification of the cartilages is known by grasping the free borders with the fingers and attempting their flexion; the rigid inflexible ossified cartilage is thus easily recognized.