A horses hoof is a complex structure made up of several parts, including the wall, sole, frog, and lateral cartilages. The hoof wall is the visible part of the hoof and is composed of a tough material called keratin, which is also known as "horn". The wall is produced continuously and must be worn or trimmed off. It does not contain blood vessels or nerves. The sole is located underneath the horses foot and is also made up of horn. It provides protection to the internal, sensitive parts of the hoof and helps absorb concussive forces. The frog is a mass of flexible material that contributes to the formation of the horses heels and is the main shock absorber. The lateral cartilages extend back and up from the inner and outer sides of the third phalanx and are flexible, but as the horse ages, they are usually ossified and replaced by bone.