| The Structure of The Horse's Hoof |
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By Stephen Champion, BSc (Hons) BVM&S MRCVS, The horse’s hoof is a complex structure that is responsible for supporting the weight of the horse above the ground. Understanding its anatomy is not easy, but anyone who keeps, trains or rides horses, will sooner or later have the address some aspect of the health of its feet. There follows a concise description of the structure of the horses’ foot, for “what one does not understand, one does not possess” (Goethe). The lower aspect of the leg of the horse is protected by the hoof, which is formed by hardening of the tissue over a greatly modified skin or dermis. The structure can be divided conveniently into four parts: the wall, the periople, the sole and the frog. The wall is the part of the hoof visible in the standing animal, whether one looks from the front or the sides. It is highest at the front part, called the toe, and lower at the sides, called the quarters, from where it forms the heels at the back of the foot. The heels divert back a short distance towards the centre of the foot as the bars, seen just adjacent to the frog when the foot is lifted up. The wall is thickest at the toe and thinner at the bars, which is important to remember when nails are being driven. The wall grows from the epithelium (the skin) at the coronet, and slides downwards towards the ground, growing at a rate of just less than 1cm per month. The periople is actually part of the wall and consists of a band of soft, rubbery horn tissue, a few millimetres thick, near the coronet. It widens further back to form the soft and familiar tissue of the bulbs of the heels and blends with the base of the frog. The sole fills the space between the wall and the frog and forms most of the under surface of the foot. It does not usually make contact with the ground, as does the wall and frog. The area between the quarters and the bars is called the seat of corn, where “corns” are sometimes seen as blood-soaked flecks due to damage to the underlying dermis. These are often caused by short shoeing, or shoes left on too long. The junction between the sole and the wall is called the white line, one of the most important structures to know about. This is a soft structure and one that is prone to penetration by stones or flints as the foot contacts rough ground. Infection can pass up the white line, following the weakest path, to burst out at the coronet. This condition has become known as “gravel” and causes enormous pain to the horse, until the infection bursts out, when the pain is reduced. Furthermore, the internal rim of the white line is where the farrier places his nails when shoeing. The nails pass obliquely through the wall to emerge a few centimetres above the shoe, where they are cut and clinched. The frog is wedged shaped and projects into the sole from behind. Its wide base is situated between the heels and spreads upwards to form the bulbs of the heels. Viewed from the bottom it has a central sulcus and at its sides it is separated from the sole by deep grooves called the collateral sulcii. In is within these that the horse, if bedded on damp bedding, or kept in persistently wet conditions, may be prone to “thrush”. This is a dark, (extremely) foul-smelling infection that generally does not cause lameness but may spread to sensitive structures beneath. A sound hoof usually requires frog contact with the ground and the soft nature of the frog absorbs much of the impact of the foot hitting the ground. In simple terms, the hoof capsule is attached to the underlying dermis by about 600 “sensitive” or dermal laminae. These interdigitate with “insensitive” horny laminae on the inside surface of the hoof wall. The laminae are like minute fingers and the structure can be imitated by interlocking the fingers of both hands together. Since the dermis is itself attached to the pedal bone (the last bone inside the foot), this means that the horse is suspended in the air by its hooves attached to its lower leg by a sensitive “quick”. If the horse is unfortunate enough to get laminitis, where this quick is inflamed, one can only imagine the pain that must be experienced and this should be addressed rapidly by the caring owner. The hoof is a flexible structure and yields under pressure when the foot hits the ground, thus dissipating concussion. The forces pass through the pedal bone to the walls of the hoof and to the frog, where the heels are spread and the frog and underlying digital cushion distort. To this end it is important that the farrier does not nail the entire shoe to the wall, otherwise the horse will develop “contracted heels” and eventually go lame. To allow further “give” in the system, the pedal bone is continued towards the hindquarters of the horse as cartilage, rather than as bone. Progressive deposition of these cartilages with age leads to “sidebone”, yet another cause of lameness. The structure of the horse’s foot is a fascinating and complex three-dimensional structure. To understand in more detail the relationship between the different anatomical features, particularly the structure and function of hoof laminae and their role in laminitis, the reader is encouraged to refer to diagrammatic texts elsewhere. |
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