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Hoof Base and Sole Physiology
The hoof consists of two digits, the outer (lateral) claw, and the inner (medial) claw. The outer wall of the claw is termed the abaxial wall and extends back to the heel bulb where it ends with a shallow indentation known as the abaxial groove. The wall folds around the toe and the inner wall, or axial wall, and meets the bulb to form a deep groove known as the axial groove. The space between the claws, separating the two heel bulbs, is known as the interdigital cleft.
Hoof horn forms a hard protective hoof wall around the more sensitive tissues and the bones within the foot. It is structurally similar to the human finger or toe nail, but has a much broader function, allowing easy mobility of the cow but providing a degree of shock absorbency and protecting the softer tissues from injury. The hoof is generally fairly resistant and protects the foot from heat, moisture, injury and infection, but many challenges are present on the farm which can reduce or limit the ability of the hoof horn to protect the inner workings of the hoof, particularly metabolic changes caused by nutrition, illness or stress that affect horn growth.
The sensitive laminar corium lines the inside of the hoof. New cells are continuous produced and are gradually pushed away from the corium. As this occurs, the cells die producing hard tissue filled with keratin, a substance also found in skin, horn and teeth. The new hard growth emerges at the papillae, small finger-like projections of the corium just below the coronary band, the point where the hoof meets the hairy skin on the cow's foot. These cells provide strength for the hoof, and extend from the papillae in a series of tubules or pipes.
Cells containing keratin also grow from the sides and the base of the papillae acting as glue which bonds the horn tubules together, forming the intertubular horn. These run vertically through the hoof and sole. The number of horn tubules is fixed at birth and so they cannot regenerate; a cow's hooves will usually get weaker or softer as she ages. Growth occurs instead by expansion of the intertubular horn as wall bands growing down the foot towards the toe.
The periople is a band of soft horn separating the hoof wall from the coronary band. It provides a protective smooth, waxy surface over the hoof, preventing excess moisture loss and the hoof from becoming brittle. This band is continuous on both claws and joins at the bulb of the heel.
Inside the hoof wall, a series of sensitive laminae run down the inside of the hoof wall, providing the dual function of being firmly attached to the underlying structures and allowing a small amount of movement and cushioning.
The horn of the sole consists of horn tubules that are softer than those of the wall. The sole corium is responsible for forming new sole horn - which grows down from the pedal bone - and where the horn of the wall meets the horn of the sole, there is a non-pigmented junction of weaker horn known as the white line. The white line has the ability to flex slightly, being softer than both the wall and sole, and runs from the heel to the toe and around the first third of the axial wall where it ceases to be a weight-bearing surface.
The sole of the hoof has no direct blood supply and relies on nutrients being passed from the corium via diffusion, making these cells very susceptible to any disturbance in the blood circulation in the corium. This partially explains why nutritional factors can have a significant impact upon foot health.
The heel bulb is continuous with the coronary band and merges with the axial and abaxial hoof wall and the sole. At the heel the corium contains fat and fibrous, elastic tissues which act as a shock absorber, compressing when weight is placed on them to absorb the impact forces. This digital cushion aids blood circulation - which plays a vital role in the formation of new horn - acting as a pump that draws blood out of the foot and into circulation as the heel makes contact with the ground. Blood vessels in the corium also expand and contract through muscle action in weight bearing; insufficient exercise can restrict these blood circulation mechanisms, impairing the formation of new horn.