The foot is composed of several structures, each of which has an important role.
Outside the Foot
Hoof Wall: The hoof wall is composed of horny material that is a specialized type of epithelium. A protein called keratin is an important component of this material. The wall continually grows and is either worn off, or is trimmed by the farrier. Growth rate is slow at about ¼ inch per month. The hoof wall has no blood supply and no nerve supply; it receives its nourishment from the tissue underlying the wall. At the back of the foot, the wall runs forward to form ridges called bars; these run parallel to the sulci (grooves on either side) of the frog. Names are given to each section of the wall: toe (front), quarters (sides) and heel (back).
Sole: The sole is similar in structure to the wall. In normal weight-bearing it does not contact the ground or contacts it to a very limited degree.
Frog: The frog is the wedge-shaped structure that is located in the angles bounded by the bars and the sole. It is composed of tissue that is resilient since it serves an important role in absorbing concussion.
Coronet or Coronary Band: This is the vascular region directly above the hoof wall from which the wall grows. It is protected by hair and a thick layer of skin.
The White Line: This "line" represents the junction between the wall and the sole, and can be visualized on the surface of the foot that contacts the ground (i.e. the ground surface).
Inside the Foot
Coffin bone: This is the main bone in the horse’s foot. It is also referred to as P3 or the third phalanx or the distal phalanx.
P2: This bone is located immediately above P3 and is also called the middle phalanx; these two bones articulate by means of a joint known as the coffin joint. Similarly, P2 articulates with P1, the bone immediately above it; this articulation is known as the pastern joint.
Navicular Bone: This small bone is located between P3 and P2 at the back of the foot, just above an important tendon called the deep digital flexor tendon (DDFT). The bone sits within a burse, which is a fluid-filled sac that serves to reduce friction between the tendon and the bone.
Dermal Tissue or Corium: This is the modified vascular tissue that is located beneath the wall and sole and frog (and the coronet) and attaches the wall and sole to the coffin bone. This tissue is vital to the healthy foot as it provides support and nourishment and growth to these overlying tissues as well as sensitivity.
Digital Cushion: This is a fibroelastic/fibrocartilage tissue that is located in the back half of the foot, and contributes to the formation of the heel. It is felt to be involved in shock absorption.
Lateral Cartilages: These fibrocartilage tissues are located upwards and back from P3 (coffin bone). Initially, they are flexible and elastic, but become boney as the horse ages.
Ligaments & Tendons: There are several ligaments within the foot that connect cartilage to bone or bone to bone within the foot. There are also tendons present that attach muscles to bone(s) in the foot (e.g. deep digital flexor tendon).
Linking Structure with Function in the Foot
As the horse’s foot impacts the ground, the frog (and the wall) makes contact and the heel expands to help distribute the concussion. The frog pushes against the digital cushion which then flattens and pushes out on the lateral cartilages. As the frog flattens it pushes against the bars of the wall, which helps to expand the heels. When the foot is lifted off the ground, these structures regain their previous shape.
As the foot impacts the ground, pressure and shape changes in the tissues in the foot will compress vessels in the foot and push the blood from the foot into the leg. When the foot is lifted, the process reverses such that blood flows into the foot. In this way, a pumping action is produced. Exercise can increase this pumping action and consequently, increase blood circulation within the foot to stimulate hoof growth and hoof health.