The changes in form take place in the following order: the body-weight falls from above upon the os coronæ, os pedis, and navicular bone, and at the moment that the foot is placed upon the ground is transmitted through the sensitive laminæ and horny laminæ to the wall. At the instant that the fetlock reaches its lowest point the os pedis bears the greatest weight. Under the body-weight the latter yields, and with the navicular bone sinks downward and backward. At the same time the upper posterior portion of the os coronæ (Fig. 90, _A_) passes backward and downward between the lateral cartilages (_a_), which project above the upper border of the wall, and presses the perforans tendon down upon the plantar cushion. The plantar cushion being compressed from above, and being unable to expand downward, is correspondingly squeezed out towards the sides and crowded against the lateral cartilages, and they, yielding, press against and push before them the wall at the quarters. The resistance of the earth acts upon the plantar surface of the hoof, and especially upon the frog, and it, widening, crowds the bars apart, and in this manner contributes to the expansion of the quarters, especially at their plantar border (see Fig. 90). The horny sole under the descent and pressure of the os pedis sinks a little—that is, the arch of the sole becomes somewhat flattened. All these changes are much more marked upon _sound unshod_ hoofs, because in them the resistance of the earth upon the sole and frog is pronounced and complete. These changes in form are more marked in front feet than in hind. In defective and diseased hoofs it may happen that at the moment of greatest weight-bearing, instead of an expansion a contraction may occur at the plantar border of the quarters. [Illustration: FIG. 90. Vertical, transverse section of a foot seen from behind: _A_, os coronæ; _B_, os navicularis; _C_, os pedis; _a_, lateral cartilage; _b_, anterior portion of fleshy frog; _c_, section of perforans tendon; _d_, suspensory ligament of the navicular bone; _l_, wall; _m_, sole; _n_, white line; _o_, frog.] Three highly elastic organs there are which play the chief part in these movements,—namely, the lateral cartilages, the plantar cushion, and the horny frog. Besides these structures, indeed, all the remaining parts of the horn capsule, especially its coronary border, possess more or less elasticity, and contribute to the above-mentioned changes of form.