The Hoof and Its Movement
The hoof wall grows from the coronary band, down the front of the pedal bone, the laminae allowing it to remain attached to the bone, and yet gradually slide down towards the ground. The rate of growth in unshod horses will adapt so that it equals the rate at which horn is worn away at the ground surface. With modern, hard road surfaces, the horn wears too fast, and growth cannot keep up with the rate of wear, so shoes can help protect the hoof from excessive wear.
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The pedal bone is suspended inside the hoof wall by the laminae. In the normal foot almost no weight is transmitted down through the sole. The bars of the hoof are largely responsible for giving support to the heels.
How The Foot Works
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As the horse takes weight, it presses down on the pedal bone, which moves within the hoof. In so doing it presses on the arteries and veins running between the bone and hoof wall and sole. The veins contain valves which allow blood to flow only one way, and the high pressure in the arteries also prevents back-flow. Thus when they are squeezed, the blood is forced through the veins, starting its journey back to the heart (often called the 'foot pump'). Because there is some resistance as the blood flows through the small vessels, this acts as an hydraulic shock absorber, reducing the concussion relayed up the limb.
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Some of the concussion of moving is also absorbed by the hoof wall, which bends as it is loaded, and by the heels of the hoof spreading. The heel expansion is a result of the load on the hoof wall, and is not related to pressure on the frog which was once thought.
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The front two thirds of the hoof contain the pedal bone, to which the hoof is attached to the laminae. This gives the wall some support when loaded. The heels however, are supported only by the horny bars of the foot, which means that the can be easily damaged and deformed if overloaded in an incorrectly shod foot.
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As the foot takes the weight, the pedal bone is pushed downwards. The heels can spread more than the front of the foot, and so the palmar aspect of the pedal bone drops more than one toe. This pulls the dorsal aspect of the hoof wall back, so that the foot takes the weight, the coronary band at the the toe moves backwards and the heels move out. As the horse walks, the weight moves forward over the foot, pulling the coronary band forward at the dorsal aspect and in at the heels. Thus there is constant flexing of the hoof wall as the horse walks.
Sagittal section of the hoof to show how the main forces act on the pedal bone, and how the bone is supported by the laminae.
Section through the laminae of the foot, showing the interlocking of the sensitive and insensitive laminae. This area forms the white zone (white line) seen in the sole of the foot.