Science Fair Project Encyclopedia
In tetrapods, claws are made of keratin, and consist of two layers. The unguis is the harder external layer which consists of keratin fibers perpendicular to the direction of growth arranged in layers at an oblique angle; and the subunguis is the softer, flaky underside layer whose grain is parallel to the direction of growth. The claw grows outward from the nail matrix at the base of the unguis, and the subunguis grows thicker while travelling across the nail bed. The unguis grows outward faster than the subunguis to produce a curve, and the thinner sides of the claw wear away faster than their thicker middle, producing a more or less sharp point. Tetrapods use their claws in many ways, commonly to grasp or kill prey, to dig, and to climb and hang.
Every so often, the growth of claws stops and restarts, as does hair. In hair, this results in the hair falling out and being replaced by a new one; in claws, this results in an abscission layer, and the old segment breaks off. This process takes several months for human thumbnails; cats are often seen working old unguis layers off on wood or on boards made for the purpose.
- Rat's Claws, also explains much about mammalian claws in general.
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