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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..."). 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 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. Grips and Grasps There are many grips and grasps that 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. Using the sense of touch users of braille are able to read without seeing this method is commonly taught to the blind of visualy impared See: braille. 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). 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: 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; 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 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 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. 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. 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. Common uses in the 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 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. Most likely derived from finding somebody with blood on their hands, indicating their guilt of a crime. See also Gallery Image:Doris Ulmann - Laborers hands.jpg|Laborer's hands, by Doris Ulmann Image:IMG 0868.JPG|Chayahuita Hands, Alto Amazonas, Loreto, Peru, 2005 Image:LeftHand.jpg|A normal human hand has four fingers and a thumb Image:CACM-film81jpg.jpg|So far, robotic hands have been far simpler than human hands ] Image:Gray220.png|Bones of the human hand Image:Pink knitting in front of pink sweatshirt.JPG|Two hands using knitting needles | |||||||
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