Vertical section through the middle of a hoof, with horny frog removed, to show the position of the bar: a, b, marks the line at which the wall bends forward and inward towards the median line of the foot to become the bar. Bar runs forward and passes imperceptibly into the sole c; a, a′, the light shading shows the part of the bar that was in contact with the horny frog.
The thickness of the wall is also variable. In front hoofs the wall is thickest at the toe, and becomes gradually thinner towards the quarters, while in hind hoofs, there is very little difference in the thickness of the wall of the toe, sides, and quarters. The more slanting half of the hoof is always the thicker; thus, for example, the outer wall of a base-wide foot is always longer and more oblique than the inner wall, and is also thicker. According to Mayer, the thickness of the wall at the toe varies from three- to five-eighths of an inch, and at the quarters from two to three-eighths of an inch. These measurements are dependent upon the size and breeding of the horse.
Fig. 35.
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.
Fig. 36.
Cross-section of keraphyllous and podophyllous laminæ (horny and fleshy leaves): a, inmost part of the solid wall; the horn-tubes approach very close to the horny leaves; b, body of the podophyllous membrane; c, horny portion of a horn-leaf directly continuous with the middle or principal layer of the wall; c′, a rudimentary horn-leaf that does not reach the body of the podophyllous membrane; c″, cross-section of horny leaves from the sides of which branch many secondary leaves (leaflets) composed of soft (young) horn-cells. These soft cellular horn-leaflets dovetail with the podophyllous or fleshy leaflets; d, podophyllous laminæ extending from the body of the podophyllous membrane; d′, podophyllous laminæ which have branched in their course to the wall, and thus given rise to c′, rudimentary horn-leaves; d″, cross-section of podophyllous leaflets extending from the sides of the podophyllous leaves; each two such leaflets secrete a keraphyllous leaflet between them; e, injected arterial vessels.