A callus (or callosity) is a toughened area of skin which has become relatively thick and hard in response to repeated friction, pressure, or other irritation. Rubbing that is too frequent or forceful will cause blisters rather than allow calluses to form. Since repeated contact is required, calluses are most often found on feet because of frequent walking. Calluses are generally not harmful, but may sometimes lead to other problems, such as skin ulceration or infection.
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http://www.aafp.org/afp/2002/0601/p2277.html corns and calluses resulting from mechanical hyperkeratosis by denise b. Freeman, d.p.m., college of podiatric medicine and surgery, des moines university–osteopathic medical center, des moines, iowa am fam physician. 2002 jun 1;65(11):2277-2280.
http://vri.cz/docs/vetmed/55-11-547.pdf a histological evaluation of bone calluses in the treatment of tibia fractures in sheep with the use of a semicircular fixator by z. Adamiak, t. Rotkiewicz faculty of veterinary medicine, university of warmia and mazury, olsztyn, poland veterinarni medicina, 55, 2010 (11): 547–550