Fig. 40.
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.
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:
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.