Claw

From Free net encyclopedia

(Redirected from Hoof)

This article relates to the claw in anatomy. For other meanings of the name Claw, see Claw (disambiguation).

A claw is a curved pointed growth found at the end of a toe or finger, or in arthropods, of the tarsus.

1 Arthropods
2 Birds
3 Tetrapods
- 3.1 Mammals
4 External links

Arthropods

The technical term for an arthropod's claw is a chela (plural chelae). Legs bearing a chela are called chelipeds. Claws are sometimes called pincers.

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Birds

A talon is the claw of a bird of prey.

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Tetrapods

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.

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Mammals

A nail is homologous to a claw, but is flatter and has a curved edge instead of a point. A nail that is big enough to walk on is called a hoof. Human nails consist only of the unguis, which we must periodically clip or file to maintain at an acceptable length, while the subunguis has mutated out. Occasionally the subunguis reappears as a sort of genetic echo from the distant past. These individuals don't have actual claws, instead the unguis appears normal with the addition of a thick growth of subunguis. One individual describes its texture as "woody". Unfortunately, as most doctors think of the growth as a symptom of a fungal infection, they are only too eager to apply painful and ultimately futile treatments.

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.

Many predatory mammals have retractile (retractable) claws that can partially hide inside the animal's paw.

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