The horny epidermal derivative covering the dorsal aspect of the terminal phalanx of each finger and toe.
The replacement of sharp claws with flattened nails in primates is considered part of the evolutionary development of a hand able to grasp objects. The nails start to develop in the fetus by the end of the third month, and are formed from the same epidermal tissue as the skin. They reach the ends of the fingers and toes about a month before birth.
The mature nail is a plate of the protein keratin which is also present in hair and skin. The pale 鈥榟alf-moon鈥?at the base 鈥?often exposed above the nail-fold only on the thumbs and big toes 鈥?marks the area of the bed of the nail where its substance is formed and from which it grows 鈥?the germinal matrix. The rest of the nail bed provides a surface for the growing nail to slide over as it advances at the rate of about 0.1 mm/day. The germinal cells continually divide to replace those which generate, and disappear into the non-cellular substance of the nail. This proliferation can be interrupted by injuries or acute illnesses, leaving grooves across the nails which gradually grow out.
ingernails grow faster than toenails. The fingernails on the right hand of a righthanded person grow faster than those on their left hand, and vice versa.
The average growth rate for nails is 0.1 mm each day (or 1 centimeter in 100 days). The exact rate of nail growth depends on numerous factors including the age and sex of the individual and the time of year. Fingernails generally grow faster in young people, in males, and in the summer.
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