The body-weight imposed at the fetlock-joint is supported, in large part, by the suspensory ligament; somewhat less weight is borne by the perforans tendon, and a still smaller amount by the perforatus. The coronary joint is supported chiefly by the perforatus, assisted by the perforans. The pedal joint is pressed forward and upward by the perforans tendon passing in a curve beneath the navicular bone. Each of these three structures bears its normal proportion of the body-weight when the three phalanges, as viewed from the side, form a continuous straight line from the fetlock-joint to the ground. In such a case the obliquity of the long pastern will be the same as that of the toe ([see Foot-Axis, p. 70]).
Fig. 19.
Right forefoot viewed from the external side: A, os coronæ; B, os pedis; C, external lateral cartilage; a, lateral pedal ligament; b, ligament uniting the lateral cartilage with the os coronæ; c, aponeurosis joining lateral cartilage and os pedis.
Raising the toe by means of a tip, a full shoe with thinned branches or a toe-calk, or paring away the quarters will tilt the os pedis backward, break the foot-axis backward in the pedal joint and to a less extent in the coronary joint, and increase the tension of the perforans tendon considerably and of the perforatus slightly. These tendons tightening behind the fetlock-joint force it forward, causing the long pastern to stand steeper, and taking some strain from the suspensory ligament. Hence, the perforans tendon is under greatest tension, and the suspensory ligament under least tension, when the foot-axis is broken strongly backward.
Shortening the toe, or raising the quarters by heel-calks or thickened branches, will tilt the os pedis forward, break the foot-axis forward in the pedal joint, and will greatly lessen the tension of the perforans tendon. The aggregate tension of perforans and perforatus tendons being diminished, the fetlock sinks downward and backward, the long pastern assumes a more nearly horizontal direction, and the tension of the suspensory ligament is increased. Thus, the perforans tendon is under least tension, and the suspensory ligament under greatest strain, when the foot-axis is broken strongly forward.
D. The Elastic Parts of the Foot.
Fig. 20.
Os pedis and inner face of one lateral cartilage; a, toe of os pedis; a′, pyramidal eminence to which the extensor tendon attaches; a″, wing of pedal bone; b, lateral cartilage; C, points of attachment of suspensory ligament of lateral cartilage; d, point of insertion of ligament to the short pastern; e, point of insertion of ligaments from navicular bone.