Although everyone knows that a hoof is a horse's foot, few people actually understand the actual mechanics of a hoof, i.e. what makes up a hoof, and how it functions. Why bother you ask? Well, since everything a horse does is on its feet (including sleeping), it is important that they be sound, because nothing limits a horse as quickly as bad feet.
First off, hoofs must be the right size to handle a horse's weight. Those that are too small are subject to contracted heels and disease, while those that are too big will make the animal clumsy and slow. In addition, the frog (underside of the heel) must be "large enough to act as a shock absorber and help pump the blood out of the legs as the horse moves. For example, in a half-ton animal, each hoof is expected to support an equal amount of weight (250 lbs of pressure). This, however, is just when it is standing in its stall. However, when in motion, a horse alternately transfers the weight from one leg to the opposite leg, thus each hoof is really forced to absorb the shock if 500 lbs of pressure every time it hits the ground.
Hoofs should be smooth, free of rings and coated with a "natural varnish." A normal front hoof should be rounded, while the rear hooves are pointed. In addition, the outside half of the foot should be a little fuller and the wall should have more "slope." In an average-sized horse, the wall should be about 3/8"-1" thick at the toe, becoming thinner as it tapers towards the heel. In addition, the sole should be concave, with the frog large and healthy, and there should be "well-developed bars to help support the wall of the hoof,' which protects both the tissue and bones. The bones are the long and short pasterns which determine the angle of the hoof, the coffin bone, and the navicular located at the rear of the foot. It is over this bone that the flexor tendons glide.
As weight is applied to the leg and pressure is sent coursing through the bones, the small bones in the knee slide over each other slightly as the pasterns sink and the hoof expands for flexibility. The frog is the key to this, forming a wedge to spread the plantar cushion (located under the bones at the heels), which then widens the lateral cartilages. Surrounding all of this is the pododerm, or quick, which (just like human toe and finger nails) contains blood vessels that nourish the foot.
The hoof wall is made up of three layers. The first is the peripole (the part with the varnish-like covering), which extends only 3/4" over the hoof, while the rest of the foot is covered by shiny horny scales (the stratum tectorium) that prevent the hoof from drying out. The second layer is the "horny wall," which is made up of the same material as hair and contains the pigment in colored feet. It also serves as a non-conductor of cold and heat. Last is the soft wall (or white line), properly called the "guideline." This is visible on the bottom of the hoof between the sole and the wall and connects the sensitive tissue to it with "insensitive laminae that inter-mesh the tissue," attaching it to the coffin bone. It is important to realize that most of the horse's weight resists on the laminae of the soft wall.
The hoof's heels are actually the first part of the foot to hit the ground, and this concussion helps them to expand. In addition, the blood forms a "hydraulic" cushion as the foot makes contact with the ground. Another interesting fact is that, although the entire hoof wall grows at the same rate, the heel wall as are always "the freshest," since they don't have far to grow. As a result, they also possess more elasticity, again, aiding in the foot's expansion. That is why improper shoeing, which inhibits this expansion, can destroy soundness of the horse.