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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. ==Cause== Normally, a callus will form on any part of the skin exposed to friction over a long period of time. For example, people often develop calluses on the middle finger of their dominant hand due to writing with a pen or pencil. Another cause is from playing string instruments like the guitar or the violin; calluses will develop on the four fingers of the hand used in holding the strings down to the fingerboard, and sometimes on the fingers of the hand used for pizzicato or strumming. Calluses are also very common on the fingers of bass guitar and double bass players who use both the pizzicato and slapping techniques. This also applies to rock climbers on almost all of their fingers. There are many activities that can result in the formation of a callus, which may even be viewed as a badge of experience and commitment to the activity. On the feet, calluses may form on the small toes due to the compression applied by tightly fitting shoes. Activities that are notorious for causing calluses include (but are not limited to) construction work, many sports, wood carving, playing musical instruments, use of chef knives, Trikke carving, rock climbing, hiking, martial arts, weight training, BMXing, dancing (especially ballet), chopping wood, and wearing high heels. Tenpin bowlers will often develop calluses on their thumbs and occasionally their middle fingers from frequent bowling. Although often found on the foot (where the most pressure and friction are applied), calluses can occur anywhere on the body as a reaction to moderate, constant "grinding" pressure. It is the natural reaction of the palmar or plantar skin. Too much friction occurring too fast for the skin to develop a protective callus will cause a blister or abrasion instead. Biologically, calluses are formed by the accumulation of terminally undifferentiated keratinocytes in the outermost layer of skin. Though the cells of calluses are dead, they are quite resistant to mechanical and chemical insults due to extensive networks of cross-linked proteins and hydrophobic keratin intermediate filaments containing many disulfide bonds. Sometimes a callus occurs where there is no rubbing or pressure. These hyperkeratoses can have a variety of causes. Some toxic materials, such as arsenic, can cause thick palms and soles. Some diseases, such as syphilis, can cause thickening of the palms and soles as well as pinpoint hyperkeratoses. There is a benign condition called ''keratosis palmaris et plantaris'', which produces corns in the creases of the fingers and non-weight bearing spaces of the feet. Some of this may be caused by actinic keratosis, which occurs due to overexposure to sun or with age and hormonal shifts. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「callus」の詳細全文を読む スポンサード リンク
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