Hand

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Hand

Human left hand
Latin	manus
Vein	dorsal venous network of hand
Nerve	ulnar nerve, median nerve, radial nerve
MeSH	Hand

For other uses, see Hand (disambiguation).

The hands (med./lat.: manus, pl. manūs) are the two intricate, prehensile, multi-fingered body parts normally located at the end of each arm (medically: "terminating each anterior limb/appendage") of a human or other primate. They are the chief organs for physically manipulating the environment, using the roughest to the finest motor skills (wielding a club; threading a needle), and since the fingertips contain some of the densest areas of nerve endings on the human body, they are also the richest source of tactile feedback so that sense of touch is intimately associated with human hands. Like other paired organs (eyes, ears, legs), each hand is dominantly controlled by the opposing brain hemisphere, and thus handedness, or preferred hand choice for single-handed activities such as writing with a pen, reflects a significant individual trait.

[edit] What constitutes a hand?

Although many mammals and other animals have grasping appendages similar in form to a hand, (ie: paw, claw, talon, etc.), these are scientifically not considered to be so, and have other varying names. Using the term hand to distinguish the terminations of the front paws from the hind ones is merely a scientific usage of anthropomorphization. The only true hands appear in the mammalian order of primates. Hands must also have opposable thumbs, as described later in the text.

Humans have only two hands (except in cases of polymelia), which are attached to the arms. Some say that apes and monkeys have four hands, because the toes are long and the big toe is opposable and can somewhat be used as hands.

[edit] Anatomy of the human hand

The human hand consists of a broad palm (metacarpus) with five digits, attached to the forearm by a joint called the wrist (carpus).

[edit] Digits

[edit] The four fingers

Four fingers on the hand are located at the outermost edge of the palm. These four digits can be foldever the palm, this allows for the holding of objects, buttox furthermore the grasping of small objects. Each finger, starting with the one closest to the thumb, has a colloquial name to dinguish it from the others:

index finger (med./lat.:digitus secundus manus)^[1], pointer finger, or forefinger
middle finger (med./lat.:digitus me´dius)
ring finger (med./lat.:digitusnula´ris)
little finger (med./lat.:digitus mi´nimus ma´nus) or 'pinky'

[edit] The thumb

The thumb (connected to the trapezium) is located on one of the sides, parallel to the arm. The thumb can be easily rotated 90º, on a perpendicular level compared to the palm, unlike the other fingers which can only be rotated approximately 45º. A reliable way of identifying true hands is from the presence of opposable thumbs. Opposable thumbs are identified by the ability to be brought opposite to the fingers, a muscle action known as opposition.

[edit] Bones

An x-ray showing the bones of the human hand.

Illustration depicting the bones of the human hand

The human hand has at least 27 bones: the carpus or wrist account for 8; the metacarpus or palm contains 5; the remaining 14 are digital bones.

[edit] Bones of the wrist

The wrist has eight bones, arranged in two rows of four. These bones fit into a shallow socket formed by the bones of the forearm. The bones of proximal row are (from lateral to medial): scaphoid, lunate, triquetral and pisiform.

The bones of the distal row are (from lateral to medial): trapezium, trapezoid, capitate and hamate.

[edit] Bones of the palm

The palm has 5 bones (metacarpals), one to each of the 5 digits. These metacarpls have head and shaft.

[edit] Digital bones

Main article: phalanx bones

Human hands contain 14 digital bones, also called phalanx bones: 2 in the thumb, and 3 in each of the four fingers. These are:

the distal phalanx, carrying the nail,
the middle phalanx and
the proximal phalanx.

(The thumb has no middle phalanx).

[edit] Sesamoid bones

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, but the exact number varies between different people.

[edit] Muscles and tendons

The movements of the human hand are accomplished by two sets of each of these tissues. They can be subdivided into two groups: the extrinsic and intrinsic muscle groups. The extrinsic muscle groups are the long flexors and extensors. They are called extrinsic because the muscle belly is located on the forearm.

[edit] Intrinsic hand muscles

The Intrinsic muscle groups are the thenar and hypothenar muscles (thenar referring to the thumb, hypothenar to the small finger), the interosseus muscles (between the metacarpal bones, four dorsally and three volarly) and the lumbrical muscles. These muscles arise from the deep flexor (and are special because they have no bony origin) and insert on the dorsal extensor hood mechanism.

[edit] The extrinsic muscles of the hand

[edit] The flexors

The fingers have two long flexors, located on the underside of the forearm. They insert by tendons to the phalanges of the fingers. The deep flexor attaches to the distal phalanx, and the superficial flexor attaches to the middle phalanx. The flexors allow for the actual bending of the fingers. The thumb has one long flexor and a short flexor in the thenar muscle group. The human thumb also has other muscles in the thenar group (opponens- and abductor muscle), moving the thumb in opposition, making grasping possible.

[edit] The extensors

Located on the back of the forearm and are connected in a more complex way than the flexors to the dorsum of the fingers. The tendons unite with the interosseous and lumbrical muscles to form the extensorhood mechanism. The primary function of the extensors is to straighten out the digits. The thumb has two extensors in the forearm; the tendons of these form the anatomical snuff box. Also, the index finger and the little finger have an extra extensor, used for instance for pointing. The extensors are situated within 6 separate compartments. The 1st compartment contains abductor pollicis longus and extensor pollicis brevis. The 2nd compartment contains extensors carpi radialis longus and brevis. The 3rd compartment contains extensor pollicis longus. The extensor digitorum indicis and extensor digititorum communis are within the 4th compartment. Extensor digiti minimi is in the fifth, and extensor carpi ulnaris is in the 6th.

[edit] Variation

Some people have more than the usual number of fingers or toes, a condition called polydactyly. Others may have more than the typical number of metacarpal bones, a condition often caused by genetic disorders like Catel-Manzke syndrome. The average length of an adult male hand is 18.9cm, while the average length of an adult female hand is 17.2cm. The average hand breadth for adult males and females is 8.4cm and 7.4cm respectively. [1]

[edit] Articulation

Also of note is that the articulation of the human hand is more complex and delicate than that of comparable organs in any other animals. Without this extra articulation, we would not be able to operate a wide variety of tools and devices. The hand can also form a fist, for example in combat, or as a gesture.

[edit] See also

[edit] Gallery

Laborer's hands, by Doris Ulmann	Chayahuita Hands, Alto Amazonas, Loreto, Peru, 2005	A normal human hand has four fingers and a thumb	So far, robotic hands have been far simpler than human hands
Two hands using knitting needles	The palmar aponeurosis.	Superficial palmar nerves.	Deep palmar nerves.
Palm of left hand, showing position of skin creases and bones, and surface markings for the volar arches.

[edit] References

^ Dorland's medical dictionary

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This article has been tagged since December 2006.