Fingertip

A finger is a type of digit, an organ of manipulation and sensation found in the hands of humans and other primates. more...

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Humans have five digits on each hand. The first digit is the thumb. Some English speakers may consider the thumb to be a type of finger, leading to the conclusion that humans have five fingers on each hand. Others may consider the term 'finger' to apply only to those four digits that are medial to the thumb, leading to the conclusion that humans have four fingers on each hand.

Chimpanzees have lower limbs that are specialized for manipulation, and (arguably) have fingers on their lower limbs as well. The term 'finger' is not applied to the digits of a plantigrade animal such as a dog, which cannot engage in fine manipulation with its forelimbs as a human can.

Anatomy

This section describes the second through fifth digits. For more information on the first digit, see thumb.

The finger is specialised for fine movement and is highly sensitive. The skin on the grasping side is thick and devoid of hair; the skin on the dorsal side is thin and mobile. The dorsal side of the finger has a nail bed at its distal end, which produces a fingernail.

Each finger is divided into three segments. Each segment contains a bone known as a phalanx (plural 'phalanges'), and the joint between each adjacent pair of segments is known as an interphalangeal joint ('IP joint'). The juncture between the finger and the palm is the site of the metacarpophalangeal joint ('MCP joint').

In humans, the distal segment is flexed by flexor digitorum profundus, and the rest are flexed by flexor digitorum superficialis. Extension is done by extensor digitorum. Lumbrical muscles flex the MCP joints. Abduction is performed using dorsal interossei muscles; adduction is performed using palmar interossei.

Each finger is supplied by four neurovascular bundles, each of which contains a vein, artery, and nerve. The arteries are supplied by either the ulnar artery or radial artery (or both), often via a superficial palmar arch and/or deep palmar arch.

Each of the arteries is an end artery; therefore, surgeons must be careful not to ligate or inject adrenaline into a digital artery. Sensation to the volar (palm) side of each finger is supplied by the ulnar nerve (medial one and a half digits) and the median nerve (lateral two and a half digits plus the thumb). These volar nerves also supply the dorsal skin of the distal segments as well (including the nail beds). The rest of the dorsal surface of each finger is supplied by the radial nerve. A whole finger may be anaesthetised with two injections of local anaesthetic, each of which paralyses two of the nerves; this procedure is known as a ring block.

In humans, the fingers have a disproportionately large allocation in the homunculus - ie, there is a large amount of brain devoted to the analysis of the sensation that arises from the fingers. The fingers are therefore highly sensitive to touch, pain, and temperature. Two-point discrimination (the ability to separately register two discrete sources of touch that are close together) is very high in the fingertips. Therefore, the fingertips may be used to read Braille, an act which would be impossible using, say, the skin of the forearm.

Read more at Wikipedia.org