_The wear of the hoof upon the shoe_ occurs as a result of the movements of the quarters. Visible indications of this are the brightly polished, often sunken places upon the bearing-surface of the ends of the branches, showing that scouring occurs between the horn and the iron. Shoes which show brightly polished places in their anterior halves have been loose. The wear of the quarters upon the shoe is not always uniform, but is usually greater on the inner than on the outer quarter, especially in base-wide feet. The degree of this wear of the hoof may be from nothing to one-fourth of an inch or more from one shoeing to the next. Finally, we should remember that this usually invisible scouring away of the hoof gradually causes the nails at the quarters to become loose, and that this is more clearly marked in the front than in the hind hoofs. G. Physiological Movements of the Hoof. (Mechanism of the Hoof.) These movements comprise all those changes of position within and of the hoof which are brought about by alternately weighting and relieving the foot, and which are manifest as changes of form of the hoof. The following changes in form of the hoof are most marked at the time that the hoof bears greatest weight,—that is, _simultaneous_ with the greatest descent of the fetlock-joint. 1. A lateral expansion over the entire region of the quarters, occurring simultaneously at the coronary and plantar borders. This expansion is small, and in general varies between one-fiftieth and one-twelfth of an inch. 2. A narrowing of the anterior half of the hoof measured at the coronary border. 3. A decrease in height of the hoof, with a slight sinking of the heels. 4. A flattening (sinking) of the sole, especially in its branches. These changes of form are much more pronounced in the half of the hoof that bears the greater weight. [Illustration: FIG. 89. 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.