nail, in anatomy, the horny outgrowth shielding the tip of the finger and the toe in humans and most other primates. The nail consists of dead cells pushed outward by dividing cells in the root, a fold of epidermis at the base of the nail (see skin). The hard material in nail cells is the tough protein material, keratin. If the root is destroyed, the nail ceases to grow. Otherwise, growth from root to tip is achieved in about four months. The small-celled and relatively bloodless tissue near the base of the nail forms a white, crescent-shaped spot called the lunula, or moon. No pigment occurs in nail cells, but since they are translucent, their appearance is pink because of blood vessels beneath. A painful inflammation (paronychium) of the fingertip may result from infection starting in a hangnail. Pressure from improperly fitting shoes may cause the large toenail to cut into the skin along its edges (the so-called ingrown toenail). Horny derivatives of the integument, homologous to the primate nail, have evolved into various structures in other animals, e.g., the hooves of horses and cattle and the claws of birds and reptiles.
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