Callosity is any thickened or hardened part of the human skin caused by pressure and friction. The term is also applied to the natural cutaneous thickenings on the buttocks of monkeys. Research Callosity
A callus is a callosity, that is a small area of skin, usually on the foot, that has become thick and hard from rubbing or pressure. Calluses may lead to other problems such as serious infection. Calluses are often caused by badly fitting shoes. The term is also applied to a new growth of osseous matter between the extremities of fractured bones, serving to unite them. Research Callus
 
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Matt and Leela Probert