Hand
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A human hand has four fingers and a thumb. |
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So far, robotic hands have been far simpler than humans ones |
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 our chief organs for physically manipulating the environment, from 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 our richest source of
tactile feedback from our environment, so that our
sense of touch is intimately associated with our 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.
Hand can also refer to an individual's
handwriting ("written in his own hand"; "a hand so free...").
Although many
mammals and other
animals have
grasping appendages similar in form to a hand, these are scientifically not considered to be so, and have other varying names, including
paws. Using the term hand is merely a scientific usage of
anthropomorphization, to distinguish the terminations of the front paws from the hind ones. The only true hands appear in the mammalian order of
primates. Hands must also feature
opposable thumbs, as described later in the text.
Grips and Grasps
There are a dizzying array of grips grasps each of us uses in our daily life: "key" grasp, "jar" grasp, "trigger grip", "hammer grip", "baseball grip", etc. Beyond everyday grips and grasps, specialists, such as violinists, have special ways of holding objects.
Braille
Because the fingers are so rich in nerve endings, the blind can read by running the fingers across raised dots. See:
braille.
The human hand consists of a broad palm (metacarpus) with five digits, attached to the forearm by a joint called the wrist (carpus).
Digits
The Four Fingers
Four
fingers on the hand are located at the outermost edge of the palm. These four digits can be folded over the palm, this allows for the holding of objects, and furthermore the grasping of small objects. Each finger, starting with the one closest to the thumb, has a colloquial name to distinguish it from the others:
* index finger, pointer finger, or forefinger
* middle finger
* ring finger
* little finger or 'pinky'
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.
Bones
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.
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.
Bones of the palm
The palm has 5 bones (
metacarpals), one to each of the 5 digits.
Digital bones
Also called phalanx bones. Human hands contain 14 of them; 2 in the thumb, and 3 in each of the four fingers, called;
* distal phalanx, carrying the nail,
* middle phalanx and
* proximal phalanx. (The thumb has no middle phalanx).
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. The
patella is the largest example of a sesamoid bone in the human body.
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.
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.
The extrinsic muscles of the hand
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.
The extensors
Located on the back of the forearm and a connected in a more complex way then 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.
Variation
Some people have more than the usual number of fingers or toes. This is normally caused by a genetic 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. Alternatively, some may have large hands, and others small hands.
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.
I know it like the back of my hand - English phrase used to say that the subject knows the matter perfectly, as if it were part of their body, or that they were born with the knowledge. Related: Second hand.
Second hand - Similar to "I know it like the back of my hand," in that it is definitely known by the subject. Similar to something being described as
second nature. Not to be confused with
second-hand goods, which have already been used before, and are being resold. In the
U.S., at least,
second hand means
indirect--almost the opposite. "She told me walking everyday is good for the brain" indicates
second hand knowledge.
A person may also describe somebody as his
right hand man, which means that he relies heavily on this person; this may derive from the importance of the right hand in right-handed individuals. The commonness of right-handedness may be the source of attitudes of importance and superiority on the right over the left by many cultures. This is exemplified by phrases such as "to be seated at the right hand of the gods when Judgment comes."
Lending a Hand - is an English phrase that means helping someone. If I ask you to "Lend me a Hand," then I am asking you to come help me.
The
hand is also an archaic unit of
measurement, and is currently used as the standard unit to measure the height of horses.
The
hand in watches is an indicator, usually made of a thin, light piece of metal, very variable in form, which moves over a graduated dial or scale.
Watches usually have three hands showing the hours, minutes and seconds.
To be caught
red-handed is to be caught in the act of doing something.
*
Dermatoglyphics*
Digit ratio*
Hand fetishism*
Handstand*
Hand walking*
Hand injuries and diseases (Handuniversity.com)
