Limb (anatomy)
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A limb (from
Old English Old English ( or , or ), or Anglo-Saxon, is the earliest recorded form of the English language, spoken in England and southern and eastern Scotland in the Early Middle Ages. It developed from the languages brought to Great Britain by Anglo-S ...
''lim'', meaning "body part") is a jointed, muscled appendage of a tetrapod vertebrate
animal Animals are multicellular, eukaryotic organisms in the Biology, biological Kingdom (biology), kingdom Animalia (). With few exceptions, animals heterotroph, consume organic material, Cellular respiration#Aerobic respiration, breathe oxygen, ...
used for weight-bearing, terrestrial locomotion and physical interaction with other objects. The distalmost portion of a limb is known as its extremity. The limbs' bony endoskeleton, known as the appendicular skeleton, is homologous among all tetrapods, who use their limbs for walking,
running Running is a method of terrestrial locomotion by which humans and other animals move quickly on foot. Running is a gait with an aerial phase in which all feet are above the ground (though there are exceptions). This is in contrast to walkin ...
and jumping, swimming, climbing, grasping, touching and striking. All tetrapods have four limbs that are organized into two bilaterally symmetrical pairs, with one pair at each end of the torso, which phylogenetically correspond to the four paired fins ( pectoral and pelvic fins) of their fish ( sarcopterygian) ancestors. The cranial pair (i.e. closer to the
head A head is the part of an organism which usually includes the ears, brain, forehead, cheeks, chin, eyes, nose, and mouth, each of which aid in various sensory functions such as sight, hearing, smell, and taste. Some very simple ani ...
) of limbs are known as the forelimbs or ''front legs'', and the caudal pair (i.e. closer to the tail or coccyx) are the hindlimbs or ''back legs''. In animals with a more erect bipedal posture (mainly hominid primates, particularly
human Humans (''Homo sapiens'') or modern humans are the most common and widespread species of primate, and the last surviving species of the genus ''Homo''. They are Hominidae, great apes characterized by their Prehistory of nakedness and clothing ...
s), the forelimbs and hindlimbs are often called upper and lower limbs, respectively. The fore-/upper limbs are connected to the thoracic cage via the pectoral/shoulder girdles, and the hind-/lower limbs are connected to the pelvis via the hip joints. Many animals, especially the arboreal species, have prehensile forelimbs adapted for grasping and climbing, while some (mostly primates) can also use hindlimbs for grasping. Some animals (
bird Birds are a group of warm-blooded vertebrates constituting the class (biology), class Aves (), characterised by feathers, toothless beaked jaws, the Oviparity, laying of Eggshell, hard-shelled eggs, a high Metabolism, metabolic rate, a fou ...
s and bats) have expanded forelimbs (and sometimes hindlimbs as well) with specialized feathers or membranes to achieve lift and fly. Aquatic and semiaquatic tetrapods usually have limb features (such as webbings) adapted to better provide propulsion in water, while marine mammals and sea turtles have convergently evolved flattened, paddle-like limbs known as flippers. 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 intrinsic fine 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
machine A machine is a physical system that uses power to apply forces and control movement to perform an action. The term is commonly applied to artificial devices, such as those employing engines or motors, but also to natural biological macromol ...
s that compensate for the lack of physical strength and endurance.


Anatomy

Limbs are attached to the torso via girdles, either the pectoral girdle for the forelimbs, or the pelvic girdle for the hindlimbs. In terrestrial tetrapods, the pectoral girdles are more mobile, floating over the rib cage connected only via the clavicles (to the sternum) and numerous muscles; while the pelvic girdles are typically fused together anteriorly via a fibrocartilaginous joint and posteriorly with the
vertebral column The spinal column, also known as the vertebral column, spine or backbone, is the core part of the axial skeleton in vertebrates. The vertebral column is the defining and eponymous characteristic of the vertebrate. The spinal column is a segmente ...
( sacrum), forming an immobile ring-like pelvis. The girdles are each connected to the corresponding limb ''proper'' via a ball-and-socket
synovial joint A synovial joint, also known as diarthrosis, joins bones or cartilage with a fibrous joint capsule that is continuous with the periosteum of the joined bones, constitutes the outer boundary of a synovial cavity, and surrounds the bones' articulati ...
. The overall patterns of forelimbs and hindlimbs are homologous among all tetrapods, as they all branched out of the same bottlenecked lineage of stegocephalians that survived the Late Devonian extinction. The body plan of tetrapod limbs are so similar (especially the pentadactyly) that they are given shared terminologies for each component of the appendicular skeleton. * 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 In classical geometry, a radius (: radii or radiuses) of a circle or sphere is any of the line segments from its Centre (geometry), center to its perimeter, and in more modern usage, it is also their length. The radius of a regular polygon is th ...
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 In anatomy, flexor is a muscle that contracts to perform flexion (from the Latin verb ''flectere'', to bend), a movement that decreases the angle between the bones converging at a joint. For example, one's elbow joint flexes when one brin ...
) aspect of the hand is known as the palm or , and that of the foot as the sole or . ** 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 keratin outgrowths such as claws and nails.


Development

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.


See also

* Anatomical terms of location * Anatomical terms of motion * Ascending limb of loop of Henle * Descending limb of loop of Henle * Orthosis * Phantom limb


References

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