In human anatomy, the upper and lower limbs are commonly known as the arms and legs respectively, although in academic usage, these terms refer specifically to the upper arm and lower leg (the lower arm and upper leg are instead called forearm and thigh, respectively). The human arms have relatively great ranges of motion and are highly adapted for grasping and for carrying objects. The extremity of each arm, known as the hand, has five opposable digits known as fingers (made up of metacarpal and metatarsal bones for hands and feet respectively) and specializes in intrinsicfine motor skills for precise manipulation of objects. The human legs and their extremities — the feet — are specialized for bipedal locomotion. Compared to most other mammals that walk and run on all four limbs, human limbs are proportionally weaker but very mobile and versatile, and the unique dexterity of the human upper extremities allows them to make sophisticated tools and machines that compensate for the lack of physical strength and endurance.[2]
The proximal half of the limb proper has one long bone, the stylopodium (plural: stylopodia), which may be the humerus of the upper arm (proximal forelimbs), or the femur of the thigh (proximal hindlimbs).
The distal half of the limb proper has two long bones, together termed the zeugopodium (plural: zeugopodia). These may be radius and ulna of the forearm, or the tibia and fibula of the shin.
The distalmost portion or extremity of the limb, i.e. the hand or foot, is known as the autopodium (plural: autopodia). Hands are technically known as the manus, and feet as the pes.
The proximal part of the autopodium, i.e. the wrist or ankle region, has many small nodular bones, collectively termed the mesopodium (plural: mesopodia). Wrist bones are known as the carpals, and ankle bones are known as the tarsals.
The middle part of the autopodium is the metapodium (plural: metapodia), composed of the slender long bones each called a metapodial. The metapodials of the hand are known as metacarpals, while the metapodials of the foot are known as metatarsals. The ventral (or flexor) aspect of the hand is known as the palm or vola, and that of the foot as the sole or planta.
The distalmost part of the autopodium are the digits (fingers or toes), which have multi-jointed phalanges and are highly mobile in most tetrapods. The ends of the digits are often protectively covered by hardened keratinoutgrowths such as claws and nails.
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.[3]
