Longitudinal section of the wall magnified. The dark stripes parallel and close together are horn-tubes; the lighter surface between the tubes represents the intertubular horn. Notice that the horn-tubes are of various diameters. The space between _a_ and _b_ represents the small tubes of the outer, darker horn of the principal (middle) layer of the wall; the space between _b_ and _c_ the lighter, inner horn of the wall; _c_, _d_, the horn separating the wall proper from the horny leaves; _d_, _e_, the horny leaves (keraphyllous tissue), on which can be seen fine, parallel, vertical stripes; in the horn-leaf at _f_, _f′_, are seen fissures passing obliquely upward and outward towards the wall.] =The Frog= (Figs. 31, _h_, 35, _k_, _l_, 38 and 39), secreted by the velvety tissue covering the plantar cushion and presenting almost the same form as the latter, lies as a wedge between the bars and between the edges of the sole just in front of the bars, with both of which structures it is intimately united. Its horn is _quite soft_ and _very elastic_. The median lacuna or cleft of the frog (Fig. 31, _l_) divides it into two branches (Fig. 31, _i_), which pass backward and outward into the horny bulbs (Fig. 31, _k_). In front of the median lacuna the two branches unite to form the _body_ of the frog (Fig. 31, _h_), which ends in a point, designated the _point_, apex, or summit of the frog. On the upper surface of the frog, directly over the median cleft of the lower surface, there is a small projection called the frog-stay (Figs. 35, _l_, 38 and 39, _b_), which fits into the median cleft of the plantar cushion. Besides, the upper surface of the frog shows many minute openings, similar to but smaller than those of the sole and coronary groove, for the reception of villi. In unshod hoofs the frog, sole, bars, and bearing-edge of the wall are on a level; that is, the plantar surface of such hoofs is perfectly flat. [Illustration: FIG. 41. Cross-section of the wall, magnified: _a_, horn-tubes; _b_, intertubular horn.] The _minute structure of the horn_ can scarcely be considered in detail in an elementary treatise such as this is. However, a few of the most important facts are as follows: