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Alimb (fromOld Englishlim, meaning "body part") is ajointed,muscledappendage of atetrapodvertebrateanimal used forweight-bearing,terrestrial locomotion and physical interaction with other objects. Thedistalmost portion of a limb is known as itsextremity. The limbs'bonyendoskeleton, known as theappendicular skeleton, ishomologous among all tetrapods, who use their limbs forwalking,running andjumping,swimming,climbing,grasping,touching andstriking.
All tetrapods have four limbs that are organized into twobilaterally symmetrical pairs, with one pair at each end of thetorso, whichphylogenetically correspond to the fourpaired fins (pectoral andpelvic fins) of theirfish (sarcopterygian) ancestors. Thecranial pair (i.e. closer to thehead) of limbs are known as theforelimbs orfront legs, and thecaudal pair (i.e. closer to thetail orcoccyx) are thehindlimbs orback legs. In animals with a more erectbipedal posture (mainlyhominidprimates, particularlyhumans), the forelimbs and hindlimbs are often calledupper andlower limbs, respectively. The fore-/upper limbs are connected to thethoracic cage via thepectoral/shoulder girdles, and the hind-/lower limbs are connected to thepelvis via thehip joints.[1] Many animals, especially thearboreal species, haveprehensile forelimbs adapted forgrasping andclimbing, while some (mostly primates) can also use hindlimbs for grasping. Some animals (birds andbats) have expanded forelimbs (and sometimes hindlimbs as well) withspecialized feathers ormembranes to achievelift andfly.Aquatic andsemiaquatic tetrapods usually have limb features (such aswebbings)adapted to better provide propulsion in water, whilemarine mammals andsea turtles haveconvergently evolved flattened,paddle-like limbs known asflippers.
Inhuman anatomy, the upper and lower limbs are commonly known as thearms andlegs respectively, although in academic usage, these terms refer specifically to theupper arm andlower leg (the lower arm and upper leg are instead calledforearm andthigh, respectively). The human arms have relatively greatranges of motion and are highly adapted for grasping and forcarrying objects. The extremity of each arm, known as thehand, has five opposabledigits known asfingers (made up of metacarpal and metatarsal bones for hands and feet respectively) and specializes inintrinsicfine motor skills forprecise manipulation of objects. The human legs and their extremities — thefeet — are specialized forbipedal locomotion. Compared to most other mammals thatwalk 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 sophisticatedtools andmachines that compensate for the lack ofphysical strength andendurance.[2]
Limbs are attached to thetorso viagirdles, either thepectoral girdle for theforelimbs, or thepelvic girdle for thehindlimbs. Interrestrial tetrapods, the pectoral girdles are more mobile, floating over therib cage connected only via theclavicles (to thesternum) and numerous muscles; while the pelvic girdles are typically fused together anteriorly viaa fibrocartilaginous joint and posteriorly with thevertebral column (sacrum), forming an immobile ring-likepelvis. The girdles are each connected to the correspondinglimbproper via aball-and-socketsynovial joint.
The overall patterns of forelimbs and hindlimbs arehomologous among all tetrapods, as they all branched out of the samebottlenecked lineage ofstegocephalians that survived theLate Devonian extinction. Thebody plan of tetrapod limbs are so similar (especially thepentadactyly) that they are given sharedterminologies for each component of theappendicular skeleton.[3]
Limb development is controlled byHox genes. All jawed vertebrates surveyed so far organize their developinglimb 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 anapical ectodermal ridge (AER) which expands in rays. AZone of Polarizing Activity (ZPA) at the rear part of the AER coordinates the differentiation of digits.[3]
