Plantar surface of right fore-hoof: _a_, _a_, bearing-surface of the toe; _a_, _b_, bearing-surface of the side walls or mammæ; _b, c_, bearing-surface of the quarters; _d_, buttress, or angle formed by wall and bar; _e_, bar; _f_, sole; _f′_, branches of the sole; _g_, white line; it passes between the sole and bars and ends at _g′_; _h_, horny frog; _i_, branches of the frog; _k_, heels, bulbs, or glomes of the hoof; _l_, median lacuna of horny frog. Between the bars and the horny frog lie the lateral lacunæ of the frog.] The hoof is divided into three principal parts, which are solidly united in the healthy foot,—namely, the =wall=, the =sole=, and the =frog=. That part of the hoof which is almost wholly visible when the foot is on the ground (Fig. 30, _b_, _c_), and which protects the foot in front and upon the sides, is known as the =wall=. In position, course, direction, and arrangement of its parts it simulates the different parts of the pododerm from which it is developed. It extends from the edge of the hair just above the coronary band to the ground; backward it gradually decreases in height (length), passes around the bulbs of the heels, and turns forward and inward (Fig. 32, _d_, _e_, and 34, _a_, _b_) to form the =bars=, which are finally lost in the edge of the sole near the summit of the frog. It thus forms at each heel an angle (Fig. 31, _d_, and 32, _d_) known as a buttress, which encloses a branch of the horny sole. Externally the wall is smooth, covered with the varnish-like periople, and presents indistinct ring-like markings (Fig. 30). Its inner surface, on the contrary, presents a great number of horn-leaves which are spoken of collectively as the _keraphyllous tissue_ (Figs. 32, _g_, and 35, _f_). The upper or =coronary border= of the wall is thin and flexible, and on its inner aspect is the =coronary groove=, into which fits the =coronary band= (Fig. 30, _f_). The lower border of the wall, called the “=bearing-edge=” or _plantar border_ (Fig. 31, _a_), is the one to which the horseshoe is fastened. By dividing a hoof from before to behind along its median line, _outer_ and _inner_ halves or _walls_ are produced, and by dividing the entire lower circumference of the wall into five equal parts or sections, a =toe=, two =side walls or mammæ=, and two =quarters= will be exhibited (Figs. 32 and 33). In order to designate these regions of the hoof still more accurately, they are spoken of as outer and inner toes, quarters, and heels. [Illustration: FIG. 32. Wall and bars seen from below: _a_, toe; _b_, side wall, or mamma; _c_, quarter; _d_, buttress; _e_, bar; _g_, horn-leaves; _h_, space occupied by the frog.]