If we carefully examine a transverse section of the horn of the wall (Fig. 41), sole, or frog, we will see with the naked eye, though much better with a magnifying glass, many minute points quite close to one another, and greatly resembling the small openings which we have seen in the coronary groove of the wall and on the upper surface of the horny sole and frog. If, now, we examine a longitudinal section of the wall (Fig. 40) or sole, we will see a number of fine, dark stripes which are straight, parallel, quite close to one another, of different widths, and which are separated by bands of lighter horn also of different widths. A thin section or slice of the wall taken at right angles to the direction of these dark lines (Fig. 41) shows us that the minute points that are visible to the naked eye, when held up to the light or moderately magnified, prove to be small openings (Fig. 41, _a_). Since these openings, shown in Fig. 41, represent the dark lines shown in Fig. 40, because an opening is found wherever there is a dark line, _we must regard all dark lines seen in longitudinal sections of wall, sole, and frog as hollow cylinders or tubes_, though they are not always hollow, but are often filled with loosely adjusted, crumbling, broken down horn-cells. The dark edges of the openings (_a_) consist of thick layers of horn-cells (tube-walls). The entire structure is called a =horn-tube=, and the lighter-colored masses of horn (Fig. 41, _b_) between the tubes are known as =intertubular horn=. =With the exception of the horny leaves of the wall and bars, all the horn of the hoof is composed of horn-tubes and intertubular horn.= The horn-tubes of the wall, sole, and frog always run downward and forward parallel to the direction of the wall at the toe,—that is, in a direction parallel with the inclination of the hoof as a whole. Although the wall, sole, and frog differ from one another considerably with respect to the size and number of the horn-tubes, the quality of the intertubular horn, and the thickness and strength of the horn-cells, these differences are only of subordinate interest or importance to the horseshoer; but he who desires to learn more of this matter is referred to the work of Leisering & Hartmann, “Der Fuss des Pferdes in Rücksicht auf Bau, Verrichtungen und Hufbeschlag,” eighth edition, Dresden, 1893. This book also treats of the variations in the quality of hoofs, which is very important for the practical horseshoer to know. It, furthermore, considers the solidity and strength of the horn of the different parts of the hoof.