The seemingly impenetrable hoof wall is actually a modified extension of skin. It grows continuously—at a rate of about 3/8 inch per month—from the coronary band corium, then it keratinizes into a hard shell of hoof wall. The specialized keratinocyte cells organize into tubules that grow downward toward the ground, and horn tissue forms perpendicular to these tubules to reinforce the horn material and provide toughness to the hoof wall. The hoof capsule is actually a laminated composite with differing zones of stiffness and elasticity.
Internal to the hoof wall lies an extensive layer of lamellae, also called insensitive and sensitive laminae, which form a large surface area that attaches the coffin bone to the hoof wall. The sensitive laminae are comprised of an interdigitating network of connective tissue, nerves and blood vessels.
The sole is also made of keratin, but it is softer and more easily worn away than the hoof wall. The white line, visualized on the bottom of the foot, is the junction between sensitive and insensitive tissues within the hoof.
The frog is a pliable structure made up of 52% keratin and 42% water in comparison to the composition of 82% keratin and 16% water for the hoof wall. Its consistency—like a rubber eraser—also helps to absorb concussion, stabilizes the hoof on impact and provides a degree of traction.
The bars of the hoof are an extension of the hoof wall that grow toward the frog to provide stability to the rear of the hoof while also dissipating some hoof-ground impact.
Healthy hoof horn is resilient and alive, expanding at the heels by much as ¼ inch with each step. The heels dampen energy while the frog and sole support the inner hoof tissues, and the toe propels the horse forward. With each foot impact, the hoof pumps blood circulation through the hoof and up the limb. The collateral cartilages are located on either side of the coffin bone, and the shock-absorbing digital cushion sits between them. There is an extensive plexus of blood vessels within the digital cushion that returns venous blood to the heart as this structure is compressed during locomotion. The more upright the foot, the greater the stiffness of the digital cushion.
As much as 90 percent of impact energy of limb loading is absorbed by components of the foot, such as the horn tubules, the frog, the digital cushion, the sensitive laminae and the collateral cartilage. These structures reduce concussion to the coffin bone and lower limb joints. Ideally, the load of landing on each foot is distributed equally across all weight-bearing parts of the hoof to minimize twisting forces on the legs and to prevent bruising within hoof structures.