Hands

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۸۵ انتشار • فعال از ۲۰۰۸

A hand is a prehensile, multi-fingered appendage located at the distal end (extremity) of the forelimbs/upper limbs of tetrapod animals — chiefly primates such as humans, chimpanzees, monkeys and lemurs — that is used primarily for grasping and object manipulation. Some non-primate arboreal vertebrates with opposable fingers such as the koala (who has two opposable thumbs on each "hand" and friction ridges extremely similar to human fingerprints), opossums, chameleons and leaf frogs are often also described as having "hands" instead of paws or feet. The giant panda (who has a "pseudothumb" made of a sesamoid wrist bone to aid grasping) and the raccoon are sometimes also described as having "hands" due to how they have adapted or improvized the upper limbs for grasping, although they actually lack opposable fingers. In comparative anatomy, the digited distal portion of an animal's forelimb has a more generic technical name — manus, which is Latin for "hand" but includes any forelimb appendages distal to the radius and ulna, such as the outer wings in birds, pterosaurs and bats, fore-flippers in secondarily aquatic tetrapods, and forepaws and lower forelegs in quadrupeds. Some evolutionary biologys use the term hand to refer more generally to any forelimb appendages with visually prominent digits — for example, in the context of whether the three digits of the bird wing involved the same homologous loss of two digits as in the dinosaur manus. The human hand usually has five digits: four fingers plus one thumb; however, these are often referred to collectively as five fingers, whereby the thumb is included as one of the fingers. Each human hand has 27 bones, not including the sesamoid bone, the number of which varies among people. These include 14 phalanges (5 proximal, 4 intermediate and 5 distal) of the fingers and thumb, 5 metacarpal bones, and the 8 carpal bones that articulate with the forearm (radius and ulna) at the wrist.