A limb (from the
Old English
Old English (, ), 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 was brought to Great Britain by Anglo-Saxon settlers in the mid-5th ...
''lim'', meaning "body part") or leg is a
jointed,
muscled
appendage that
tetrapod vertebrates use for
weight-bearing In orthopedics, weight-bearing is the amount of weight
In science and engineering, the weight of an object is the force acting on the object due to gravity.
Some standard textbooks define weight as a vector quantity, the gravitational force acti ...
and
terrestrial locomotion such as
walking,
running
Running is a method of terrestrial locomotion allowing humans and other animals to move rapidly on foot. Running is a type of gait characterized by an aerial phase in which all feet are above the ground (though there are exceptions). This i ...
and
jumping, for
paddle-swimming, or for
grasping and
climbing
Climbing is the activity of using one's hands, feet, or any other part of the body to ascend a steep topographical object that can range from the world's tallest mountains (e.g. the eight thousanders), to small boulders. Climbing is done ...
. The distalmost portion of a limb is known as its extremity. The limbs'
bony endoskeleton, known as the
appendicular skeleton
The appendicular skeleton is the portion of the skeleton of vertebrates consisting of the bones that support the appendages. There are 126 bones. The appendicular skeleton includes the skeletal elements within the limbs, as well as supporting sh ...
, is
homologous among all tetrapods.
All tetrapods have four limbs that are organized into two
bilaterally symmetrical
Symmetry in biology refers to the symmetry observed in organisms, including plants, animals, fungi, and bacteria. External symmetry can be easily seen by just looking at an organism. For example, take the face of a human being which has a pl ...
pairs, with one pair at each end of the
torso
The torso or trunk is an anatomical term for the central part, or the core, of the body of many animals (including humans), from which the head, neck, limbs, tail and other appendages extend. The tetrapod torso — including that of a hu ...
. The
cranial
Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...
pair are known as the
forelimbs or ''front legs'', and the
caudal pair the
hindlimbs or ''back legs''. In animals with more
upright posture
Bipedalism is a form of terrestrial locomotion where an organism moves by means of its two rear limbs or legs. An animal or machine that usually moves in a bipedal manner is known as a biped , meaning 'two feet' (from Latin ''bis'' 'double' ...
(mainly
hominid primate
Primates are a diverse order of mammals. They are divided into the strepsirrhines, which include the lemurs, galagos, and lorisids, and the haplorhines, which include the tarsiers and the simians ( monkeys and apes, the latter includin ...
s, particularly
human
Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedalism and exceptional cognitive skills due to a large and complex brain. This has enabled the development of advanced tools, cultu ...
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
shoulder girdle
The shoulder girdle or pectoral girdle is the set of bones in the appendicular skeleton which connects to the arm on each side. In humans it consists of the clavicle and scapula; in those species with three bones in the shoulder, it consists ...
s, and the hind-/lower limbs are connected to the
pelvis via the
hip joints. Many animals, especially the
arboreal species, have
prehensile
Prehensility is the quality of an appendage or organ that has adapted for grasping or holding. The word is derived from the Latin term ''prehendere'', meaning "to grasp". The ability to grasp is likely derived from a number of different orig ...
forelimbs adapted for
grasping and
climbing
Climbing is the activity of using one's hands, feet, or any other part of the body to ascend a steep topographical object that can range from the world's tallest mountains (e.g. the eight thousanders), to small boulders. Climbing is done ...
, while some (
birds and
bats) have expanded forelimbs (and sometimes hindlimbs as well) with
specialized feathers or
membranes to achieve
lift and
fly
Flies are insects of the order Diptera, the name being derived from the Greek δι- ''di-'' "two", and πτερόν ''pteron'' "wing". Insects of this order use only a single pair of wings to fly, the hindwings having evolved into advanced ...
. Some animals (mostly primates) can also use hindlimbs for grasping.
In the
human body, the upper and lower limbs are commonly known as the
arms and
legs, respectively (although in academic definitions, these latter terms actually refer only to the
upper arm
In human anatomy, the arm refers to the upper limb in common usage, although academically the term specifically means the upper arm between the glenohumeral joint (shoulder joint) and the elbow joint. The distal part of the upper limb between t ...
and
lower leg). 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, specializes in
intrinsic and
digital
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
tool
A tool is an object that can extend an individual's ability to modify features of the surrounding environment or help them accomplish a particular task. Although many animals use simple tools, only human beings, whose use of stone tools dates b ...
s and
machines that compensate for the lack of
physical strength and
endurance.
Terminology
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
The carpal bones are the eight small bones that make up the wrist (or carpus) that connects the hand to the forearm. The term "carpus" is derived from the Latin carpus and the Greek καρπός (karpós), meaning "wrist". In human anatomy, the ...
and
digits. As
metapodials, the
metacarpals and
metatarsals are analogous to each other.
Development
Limb development is controlled by
Hox genes. All jawed vertebrates surveyed so far organize their developing
limb bud
The limb bud is a structure formed early in vertebrate limb development. As a result of interactions between the ectoderm and underlying mesoderm, formation occurs roughly around the fourth week of development. In the development of the human em ...
s 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
The zone of polarizing activity (ZPA) is an area of mesenchyme that contains signals which instruct the developing limb bud to form along the anterior/posterior axis. Limb bud is undifferentiated mesenchyme enclosed by an ectoderm covering. Even ...
(ZPA) at the rear part of the AER coordinates the differentiation of digits.
See also
*
Anatomical terms of location
Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position p ...
*
Anatomical terms of motion
*
Orthosis
Orthotics ( el, Ορθός, translit=ortho, lit=to straighten, to align) is a medical specialty that focuses on the design and application of orthoses, or braces. An is "an externally applied device used to influence the structural and functi ...
*
Phantom limb
*
Descending limb of loop of Henle
Within the nephron of the kidney, the descending limb of loop of Henle is the portion of the renal tubule constituting the first part of the loop of Henle.
Physiology
The permeability is as follows:
Also, the medullary interstitium is highly ...
*
Ascending limb of loop of Henle
References
{{Authority control