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The five-rayed anterior limbs of terrestrial vertebrates can be derived phylogenetically from the pectoral fins of fish. Within the taxa of the terrestrial vertebrates, the basic pentadactyl plan, and thus also the fingers and phalanges, undergo many variations.[3]Morphologically the different fingers of terrestrial vertebrates are homolog. The wings of birds and those of bats are not homologous, they are analogue flight organs. However, the phalanges within them are homologous.[4]

Chimpanzees have lower limbs that are specialized for manipulation, and (arguably) have fingers on their lower limbs as well. In the case of Primates in general, the digits of the hand are overwhelmingly referred to as "fingers".[5][6] Primate fingers have both fingernails and fingerprints.[7]

Research has been carried out on the embryonic development of domestic chickens showing that an interdigital webbing forms between the tissues that become the toes, which subsequently regresses by apoptosis. If apoptosis fails to occur, the interdigital skin remains intact. Many animals have developed webbed feet or skin between the fingers from this like the Wallace's flying frog.[8][9][10]Usually humans have five digits,[11] the bones of which are termed phalanges,[2] on each hand, although some people have more or fewer than five due to congenital disorders such as polydactyly or oligodactyly, or accidental or intentional amputations. The first digit is the thumb, followed by index finger, middle finger, ring finger, and little finger or pinkie. According to different definitions, the thumb can be called a finger, or not.

English dictionaries describe finger as meaning either one of the five digits including the thumb, or one of the four excluding the thumb (in which case they are numbered from 1 to 4 starting with the index finger closest to the thumbEvolution has provided the human body with two distinct features: the specialization of the upper limb for visually guided manipulation and the lower limb's development into a mechanism specifically adapted for efficient bipedal gait.[10] While the capacity to walk upright is not unique to humans, other primates can only achieve this for short periods and at a great expenditure of energy.[11]

The human adaption to bipedalism has also affected the location of the body's center of gravity, the reorganization of internal organs, and the form and biomechanism of the trunk.[12] In humans, the double S-shaped vertebral column acts as a great shock-absorber which shifts the weight from the trunk over the load-bearing surface of the feet. The human legs are exceptionally long and powerful as a result of their exclusive specialization for support and locomotion—in orangutans the leg length is 111% of the trunk; in chimpanzees 128%, and in humans 171%. Many of the leg's muscles are also adapted to bipedalism, most substantially the gluteal muscles, the extensors of the knee joint, and the calf muscles.[13]

The thumb (connected to the trapezium) is located on one of the sides, parallel to the arm.

The palm has five bones known as metacarpal bones, one to each of the five digits. Human hands contain fourteen digital bones, also called phalanges, or phalanx bones: two in the thumb (the thumb has no middle phalanx) and three in each of the four fingers. These are the distal phalanx, carrying the nail, the middle phalanx, and the proximal phalanx. Joints are formed wherever two or more of these bones meet. Each of the fingers has three joints:

metacarpophalangeal joint (MCP) – the joint at the base of the finger

proximal interphalangeal joint (PIP) – the joint in the middle of the finger

distal interphalangeal joint (DIP) – the joint closest to the fingertip.

Sesamoid bones are small ossified nodes embedded in the tendons to provide extra leverage and reduce pressure on the underlying tissue. Many exist around the palm at the bases of the digits; the exact number varies between different people.

The articulations are: interphalangeal articulations between phalangeal bones, and metacarpophalangeal joints connecting the phalanges to the metacarpal bones. 1

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