The =horn sole= (Fig. 31, _f_, and Fig. 35, _g_) is secreted by the velvety tissue of the sole. A sole from which the loose flakes of old horn have been removed is about as thick as the wall. It covers the under surface of the foot, and presents upon its upper surface a convexity which exactly fits into the concavity on the under surface of the os pedis. This upper surface is thickly covered by a multitude of minute funnel-shaped openings for the reception of the villi of the velvety tissue of the sole (Fig. 37). The lower surface of the sole is more or less concave, rough, uneven, and often covered by loose scales of dead horn. Behind, the sole presents a triangular opening whose borders lie partly in contact with the horny frog and partly with the bars. This opening or re-entering angle divides the sole into a _body_ (Fig. 31, _f_) and two wings or _branches_ (Fig. 31, _f′_). The outer border of the sole unites through the medium of the =white line= with the lower part of the inner surface of the wall,—that is, with the keraphyllous layer of the wall. This =white line= (Figs. 31, _g_, and 35, _h_), of so much importance to the horseshoer, is formed by the horn-leaves, and by those short plugs of tubular horn which are secreted by the villi that are always found at the lower ends of the fleshy leaves. The white line may be said to exist wherever the horn-leaves can be discerned upon the plantar surface of the hoof. It not only passes around the circumference of the sole from heel to heel, but may be followed forward from the buttresses along the bars almost to the summit of the frog. The horn of the white line is soft, unpigmented (white), and possesses so very little resistance (strength) that it is often found crumbling or even absent in places. The visible part of the white line is usually of a grayish-black color, owing to the working in from below of dirt and liquid manure, and to staining by rust from the nails. =The white line is very important, since it serves as the point from which we judge of the thickness of the wall, and because the horseshoe nail should penetrate it.= [Illustration: FIG. 39. A horny frog cut vertically and lengthwise through its middle: _a_, upper surface; _b_, frog-stay; _c_, median lacuna of frog, which at _c′_, is overlaid with superposed layers of horn.] [Illustration: FIG. 40.