Hand

The hand is a portion of the arm or anterior limb of a human or other primate , at where the appendage terminates. This part of the limb isespecially used in grasping and holding. The left hand is the mirror image of the right hand.

What constitutes a hand?

Although many mammals and other animals have graspingappendages 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 pawsfrom the hind ones. The only true hands appear in the mammalian order of primates .Hands must also feature opposable thumbs , as described later in thetext.

Structure of the hand

The 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

The four fingers on the hand are located at the outermost edge of the palm. Thesefour 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 or pointer finger
• middle finger
• ring finger
• little finger or pinky

The thumb

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

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 theforearm.

Bones of the palm

The palm has 5 bones, one to each of the 5 digits.

Digital bones

Also called phalanges, 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 theunderlying 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 extonsors. They are calledextrinsic because the muscle belly is located on the forearm.

Intrinsic hand muscles

The Intrinsic muscle groups are the thenar and hypothenar muscles (thenarreferring to the thumb, hypothenar to the little finger), the interosseus muscles (between the metacarpal bones, four dorsallyand three volarly) and the lumbrical muscles. These muscles arise from the deep flexor (and are special because they have no bonyorigin) 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 thefingers. The deep flexor attaches to the distal phalanx, and the superficial flexor attaches to the middle phalanx. The flexorsallow for the actual bending of the fingers. The thumb has one long flexor and a short flexor in the thenar muscle group. Thehuman thumb also has other muscles in the thenar group (opponens- and abductor muscle), moving the thumb in opposition, makinggrasping 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. Thetendons unite with the interosseous and lumbrical muscles to form the extensorhood mechanism. The primary function of theextensors 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 fingerhave an extra extensor, used for instance for pointing.

Articulation

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

See also: Common uses in of the word hand in the English language , hand (clock) , hand (measurement) , hand (mechanisms) , hand (language) .

Common uses in English language

I know it like the back of my hand - English phrase used to say that the subject knows the matter perfectly,as if it was 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 havealready 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 thisperson.

A hand is also:

• A measurement of length, primarily used for the height of horses , see hand (unit of length) .
• Something that is like a human hand, such as the rotating pointer on the face of a clock with analog display or ona gauge or dial ( hand(mechanisms) ).
• An action: "Hand me the remote".
• Learned Hand (1872-1961), a jurist who became chief judge of the U.S. 2nd District Court of Appeals.
• A hand (language) used in print materials, which is used to call attention to a particular paragraph or point; a lateral direction, (for example,"there is a button malfunctioning on the right hand side of the remote control."), hand (direction) .
• Someone's own style of handwriting or a particular type of handwriting,such as Italic or Cursive .