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The hand (med./lat.: manus) is the portion of the arm or anterior limb of a human or other primate, where the appendage terminates. This part of the limb is especially used in grasping and holding.
What constitutes a hand?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. Human anatomy of the handThe human hand consists of a broad palm (metacarpus) with five digits, attached to the forearm by a joint called the wrist (carpus). DigitsThe Four FingersFour 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:
The thumbThe 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 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. The carpals, metacarpals, and phalanges in the human hand are visible in this x-ray.BonesThe 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 wristThe 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 the distal row are (from lateral to medial): trapezium, trapezoid, capitate and hamate. Bones of the palmThe palm has 5 bones (metacarpals), one to each of the 5 digits. Digital bonesAlso called phalanx bones. Human hands contain 14 of them; 2 in the thumb, and 3 in each of the four fingers, called;
(The thumb has no middle phalanx). Sesamoid bonesSesamoid 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 tendonsThe 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 musclesThe 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 handThe flexorsThe 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 extensorsLocated 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. VariationSome people have more than the usual number of fingers or toes, this is normally caused by a genetic condition called Polydactyly. ArticulationTwo hands using knitting needlesAlso 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. Common uses in the English languageI 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, deriving from the importance and superiority place on the right over the left by many civilizations. Exemplified by phrases such as 'he is my right hand' and '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 and literally "lend your hand". The hand is also an archaic unit of measurement. Phalanx BonesThe phalanges in a human hand. Bones of the human hand.The name Phalanges is commonly given to the bones that form fingers and toes. In primates such as humans and monkeys, the thumb and big toe have two phalanges, while the other fingers and toes consist of three. The phalanges do not really have individual names but are named after the digit, and their distance from the body. Distal phalanges are at the tips of the fingers and toes, the proximal phalanges are closest to the hand (or foot) and articulate with the metacarpals or metatarsals. Middle phalanges are between the distal and proximal. The thumb and big toe do not have middle phalanges. The phalanges of the foot correspond with those of the hand. They differ from them in their size (the bodies being much reduced in length) and being laterally compressed. First Row, The body of each is compressed from side to side, convex above, concave below. The base is concave; and the head presents a trochlear surface for articulation with the second phalanx. Second Row, The phalanges of the second row are remarkably small and short, but rather broader than those of the first row. The ungual phalanges, in form, resemble those of the fingers; but they are smaller and are flattened from above downward; each presents a broad base for articulation with the corresponding bone of the second row, and an expanded distal extremity for the support of the nail and end of the toe. The phalanges are each ossified from two centers: one for the body, and one for the base. The center for the body appears about the tenth week, that for the base between the fourth and tenth years; it joins the body about the eighteenth year. See also
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