The outer wall of the hoof has been removed by cutting vertically through the middle of the toe, down to the upper surface of the sole, then horizontally backward into the quarter, and, finally, upward through the quarter: _a_, perioplic horn-band; _b_, coronary groove; it turns inward and forward at _c_ to form the upper border of the bar; _d_, surface of section of the wall at the toe; _d′_, at the quarter; _e_, surface of horizontal section of the wall near its lower border; _f_, keraphyllous layer of the wall; at _f′_ it turns forward and inward to cover the bar; _f″_, horny leaves standing free and passing insensibly into the white horn of the middle layer or true wall; _g_, horny sole; _h_, white line; _i_, small horn-spur in middle of toe; _k_, part of horny frog which is in intimate union with the upper edge of the bar; _l_, frog-stay of horny frog; it divides the trough-like depression of the upper surface of the frog into _m_, the two upper channels of the frog.] The horn wall is composed of _three superposed layers_. These from without to within are: (1) the =periople=, secreted by the perioplic band. It is very thin, glistening, and varnish-like in appearance, and covers the entire outer surface of the wall, except where it has been removed by the rasp, and prevents rapid evaporation of moisture from the horn. (2) The middle or =protective layer= (Fig. 35, _d_) is the thickest, strongest, and most important of the three layers; it forms the principal mass of the wall, and is developed or secreted by the coronary band, which fits into the coronary groove. There are in the coronary groove a great number of small, funnel-shaped openings into which project the horn-producing villi or papillæ of the coronary band. (3) The =inner layer= or =keraphyllous layer= (Fig. 35, _f_) consists of prominent, parallel horn-leaves lying side by side over the entire inner surface of the middle layer of the wall, and continuing beyond the buttresses to the ends of the bars (Fig. 35, _f′_). This layer of horn-leaves (keraphyllous layer) has in a general way about the same shape and arrangement as the layer of fleshy leaves (podophyllous layer) which secretes it; for the horn-leaves fit in with the fleshy leaves in such a way that every fleshy leaf is embraced by two horn-leaves, and every horn-leaf by two fleshy leaves (Fig. 36). The keraphyllous layer and the horn of the inmost part of the middle or protective layer are always white, even in pigmented (colored) hoofs. [Illustration: FIG. 36.