Hoof - Anatomy & Physiology
The keratin in the epidermis, when thickened and cornified, is referred to as horn. Horn is particularly resistant to mechanical and chemical damage. The outer surface of the hoof is composed of horn.
Structure and Function
The epidermis of the horny hoof is termed the coronary epidermis. Its surface has a plantar aspect and newly formed hoof grows distally towards the plantar surface of the structure. Hoof wall is generally 5 - 10 mm in thickness and consists of 3 layers:
- A thin outer layer of shiny, dense horn, which effectively seals the hoof against dehydration and penetration.
- A thicker, intermediate layer of amorphorous horn, interspersed with tubular horn which provides reinforcement. This layer makes up the bulk of the hoof.
- An inner, lamellar layer, where epidermis and dermis interdigitate, anchoring the dead portion of the hoof to the living surface of the third phalanx.
The pigmentation in the outer layer of hooves is derived from melanocytes in the coronary epidermis. Deeper layers contain little melanin and therefore appear lightly coloured or white. The unpigmented layer of keratin forms a white line on the sole of the hoof, which delineates the sole from the wall when the plantar aspect is viewed.
The keratin in the sole is formed by the epidermis of the plantar aspect of the third phalanx and reaches a maximum thickness of 1 cm. It is less resistant to abrasion than that of the outer layer of the hoof. The central frog of the sole of a horse is softer and less rigid than the keratin of the wall of the hoof. Its elasticity is particulary important in the horse as pressure on the frog changes the angle of the walls when the horse stands on that foot. The change in angle, termed the hoof mechanism increases the blood circulation in the hoof, increasing the supply of nutrients to the coronary epidermis. This is closely correlated to hoof wall quality.
In other hooved species, the bulb of the heel is analagous to the frog, with a very similar function.