The walls are considered as a protective shield covering the sensitive internal hoof tissues (like the exoskeleton of arthropods), as a structure devoted to dissipating the energy of concussion, and as a surface to provide grip on different terrains. They are elastic and very tough, and vary in thickness from 6 to 12 mm. The walls are composed of three distinct layers: the pigmented layer, the water line and the white line.
The pigmented layer is generated by the coronet, and its color is just like that of the coronet skin from which it is derived. If the coronet skin has any dark patch, the walls show a parallel pigmented line, from the coronet to the ground, showing the wall’s growth direction. This layer has predominately protective role, and is not as resistant to ground contact, where it can break and flake away.
The water line is built up by the coronet and by the wall’s corium (the living tissue immediately beneath the walls). Its thickness increases proportionally to the distance from the coronet and, in the lower third of the walls, is thicker than the pigmented layer. It is very resistant to contact to the ground, and it serves mainly a support function.
The white line is the inner layer of the wall. It is softer and fibrous in structure and light in color; white in a freshly trimmed hoof, yellowish or gray after exposure to air and dirt. From the underside of the healthy hoof, it is seen as a thin line joining the sole and the walls. The white line grows out from the laminar connections. Any visible derangement of the white line indicates some important derangement of laminar connections that fix the walls to the underlying P3 bone. Since the white line is softer than both the walls and the sole, it wears fast where it appears on the surface; it appears as a subtle groove between the sole and the walls, often with some debris or sand inside.
The three layers of the wall merge in a single mass and they grow downwards together. If the wall does not wear naturally, from sufficient movement on abrasive terrains, then it will protrude from the solar surface. It then becomes prone to breakage, and the healthy hoof will self-trim, by breaking or chipping off.
When a horseshoe is applied, it is fixed to the wall. Nails are driven in, oblique to the walls. They enter the wall at the outside edge of the white line and they emerge at the wall’s surface, about 15 to 20 mm from the base of the wall.
The wall is anatomically analogous to the human finger or toe nail.
The frog is a V shaped structure that extends forwards across about two-thirds of the sole. Its thickness grows from the front to the back and, at the back, it merges with the heel periople. In its midline, it has a central groove (sulcus), that extends up between the bulbs.
It is dark gray-blackish in color and of a rubbery consistency, suggesting its role as shock absorber and grip tool on hard, smooth ground. Actually, the frog acts like a pump to move the blood back to the heart, a great distance from the relatively thin leg to the main organ of the circulatory system.
In the stabled horse, the frog does not wear, but degrades, due to bacterial and fungal activity, to an irregular, soft, slashed surface. In the free-roaming horse, it hardens into a callous consistency with a near-smooth surface.
It is anatomically analogous to the human fingertip.
The sole has a whitish-yellowish, sometimes grayish color. It covers the whole space from the perimeter of the wall to the bars and the frog, on the underside of the hoof. Its deep layer has a compact, waxy character and it is called ‘live sole’. Its surface is variable in character as a result of ground contact. If there is no contact, as in shod hooves or when the walls are too long or the movement poor, the lower surface of the sole has a crumbly consistency, and it is easily abraded by scratching it with a hoofpick. Conversely, it has a very hard consistency, with a smooth, bright surface, when there is a consistent, active contact with the ground. The front portion beneath the front of the pedal bone is called the ‘sole callus’.
A stone bruise affects the sole of the horse’s foot. It is often caused by a horse treading on a stone or sharp type of object, landings from high jumps and excessive exposure to snow. A major symptom is lameness.
Bars are the inward folds of the wall, originating from the heels at an abrupt angle. The strong structure built up by the extremity of the heel and of the bar is named the ‘heel buttress’. The sole between the heel walls and the bars is named the ‘seat of corn’, and it is a very important landmark used by natural hoof trimmers to evaluate the correct heel height. The bars have a three-layer structure, just like the walls (see above). When overgrown, they bend outwards and cover the lower surface of the sole.
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