A horse hoof is the lower extremity of each leg of a horse, the part that makes contact with the ground and carries the weight of the animal. It is both hard and flexible. It is a complex structure surrounding the distal phalanx of the 3rd digit (digit III of the basic pentadactyl limb of vertebrates, evolved into a single weight-bearing digit in horses) of each of the four limbs, which is covered by soft tissue and keratinised (cornified) matter.
Anatomy
The hoof is made up of two parts. The outer part, called the hoof capsule, is composed of various cornified specialized structures. The inner, living part of the hoof is made up of soft tissues and bone. The cornified materials of the hoof capsule differ in structure and properties. Dorsally, it covers, protects, and supports P3 (also known as the coffin bone, pedal bone, or PIII). Palmarly/plantarly, it covers and protects specialised soft tissues, such as tendons, ligaments, fibro-fatty and/or fibrocartilaginous tissues, and cartilage. The upper, almost circular limit of the hoof capsule is the coronet (also called the coronary band), which is at an angle to the ground of roughly similar magnitude in each pair of feet (i.e., fronts and backs). These angles may differ slightly from one horse to another, but not markedly. The walls of the hoof originate from the coronary band. Walls are longer in the dorsal portion of the hoof (toe), intermediate in length in the lateral portion (quarter), and short in the palmar/plantar portion (heel). Heels are separated by an elastic, resilient structure named the 'frog'. In the palmar/plantar part of the foot, above the heels and the frog, there are two oval bulges named the 'bulbs'. When viewed from the lower surface, the hoof wall's free margin encircles most of the hoof. The triangular frog occupies the center area. Lateral to the frog are two grooves, deeper in their palmar portion, named 'collateral grooves' or sulci. At the heels, the palmar/plantar portions of the walls bend inward sharply, following the external surface of the collateral grooves to form the bars. The lower surface of the hoof, from the outer walls and the inner frog and bars, is covered by an exfoliating keratinised material, called the 'sole'. Just below the coronet, the walls are covered for about an inch by a cornified, opaque 'periople' material. In the palmar/plantar part of the hoof, the periople is thicker and more rubbery over the heels, and it merges with frog material. Not all horses have the same amount of periople. Dry feet tend to lack this substance, which is sometimes substituted with a hoof dressing.
Characters and functions of the external hoof structures
The walls 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 or more. The walls are composed of three distinct layers: the pigmented layer, the water line, and the white line, all originating from the coronary papillae. The pigmented layer's color is just like that of the coronet skin from which it is derived. If the coronet skin has any dark patches, the walls show a corresponding pigmented line, from the coronet to the ground, showing the wall's growth direction. This layer has a predominantly protective role and is not as resistant to ground contact, which can cause it to break and flake away. The water line's 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 with 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 form a single mass, growing downwards together. If the wall does not wear naturally from sufficient movement on abrasive terrain, 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 fingernail or toenail.
The frog
The frog is a V-shaped structure that extends forward across about two-thirds of the sole. Its thickness grows from the front to the back; at the back, it merges with the heel periople. It has a central groove (sulcus) in its midline that extends up between the bulbs. It is dark gray-blackish in color and of a rubbery consistency, suggesting its role as a shock absorber and grip tool on hard, smooth ground. The frog also 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. For good health, the horse requires dry areas to stand. If exposed to consistently watery environments, the frog can develop a bacterial infection called thrush. The frog is anatomically analogous to the human fingertip.
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