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. (January 2018)
A limb (from the Old English lim), or extremity, is a jointed, or prehensile, appendage of the human or other animal body. In the human body, the arms and the legs are commonly called the upper limbs and lower limbs respectively, to include part of the shoulder and hip girdles. Arms and legs are connected to torso or trunk.
Many animals use limbs for locomotion, such as walking, running or climbing and swimming. Some animals can use their forelimbs (which are homologous to arms in humans) to carry and manipulate objects, while some can use them to achieve flight. Some animals can also use hind limbs for manipulation.
Human legs and feet are specialized for two-legged locomotion – most other mammals walk and run on all four limbs. Human arms are weaker, but very mobile, allowing them to reach at a wide range of distances and angles, and end in specialized hands capable of grasping and fine manipulation of objects. Though human dexterity is relatively unique, grasping behavior is widespread among tetrapods.
The overall patterns of the forelimbs and hindlimbs are so similar ancestrally, and branch out in similar ways; that they are given shared names. Limbs are attached to the pectoral girdle or pelvic girdle. The one bony element of the upper limb is the stylopodium, the two bones of the lower limb are the zeugopodium. The distal portion of the limbs, that is, the hands or feet, are known as autopodia. Hands are technically known as the manus, and feet as the pes, which are both composed of carpals and digits. As metapodials, the metacarpals and metatarsals are analogous to each other.
Limb development is controlled by Hox genes. All jawed vertebrates surveyed so far organize their developing limb buds in a similar way. Growth occurs from proximal to distal part of the limb. On the distal end, the differentiation of skeletal elements occurs in an apical ectodermal ridge (AER) which expands in rays. A Zone of Polarizing Activity (ZPA) at the rear part of the AER coordinates the differentiation of digits.