Transverse vertical section through the middle of a right fore shod hoof of base-wide form, viewed from behind. The outer wall having been insufficiently lowered has caused increased wear of the underlying branch of the shoe: a, wear of inner branch (beneath the relatively short wall); b, greater wear of outer branch beneath the relatively long wall; c, the horn between the dotted line and the shoe represents the surplus length of this outer wall.
A hoof while supporting the body-weight has a different form, and the tissues enclosed within it a different position, than when not bearing weight. Since loading and unloading of the foot are continually alternating, the relations of internal pressure even in the standing animal are continuously changing, so that, strictly speaking, the hoof is never at rest.
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.
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.
In order to maintain the elastic tissues of the foot in their proper activity, regular and abundant exercise, with protection against drying out of the hoof, are absolutely necessary, because the movements of the different structures within the foot and the changes of form that occur at each step are indispensable in preserving the health of the hoof. Long-continued rest in the stable, drying out of the hoof, and shoeing decrease or alter the physiological movements of the foot, and these lead under certain conditions to foot diseases, with which the majority of horse owners are entirely unacquainted.
As an outward, visible indication of the mobility of the quarters upon the shoe we may point to the conspicuous, brightly polished, and often sunken spots, or grooves, upon the ends of the branches. They are produced partly by an in-and-out motion of the walls at the quarters, and partly by a forward and backward gliding of the quarters upon the shoe.