Hand
From Encyclopædia
The human hand is a specialized
organ composed of a complex arrangement of bones, muscles, and tendons that permits movements suitable for a wide variety of tasks. Highly developed thumb opposability, unique to the human hand, allows humans to grasp, pull, and push in a way that sets them apart from lower animals. In addition, the fingers contain a large concentration of nerves useful for sensory evaluation. The hand includes 27 bones and over 20
joints. These bones are divided into three groups: 8 carpal bones in the wrist; 5 metacarpal bones, which form the knuckles; and 14 phalanges, or finger bones, 3 for each finger and 2 for the thumb. The movement of the hand involves the use of 33 different muscles. Large muscles located in the forearm direct most hand action, reaching the fingers by long tendons that travel to the fingers through a bracelet-like bone structure in the wrist called the carpal tunnel. Eleven muscles in the hand itself help to flex and extend fingers as well as to fan fingers apart and bring them together (see HUMAN BODY).It is estimated that the human hand makes over 1,000 different movements in a single day. This continuous activity often
leads to repetitive motion injuries, as in the
case of CARPAL TUNNEL SYNDROME (CTS), in which the tendons in the carpal tunnel swell and compress nerves, resulting in hand numbness. De Quervain's disease is similar to CTS except that it causes numbness in the thumb, preventing thumb movement. Hands can also be used as indicators of disease. For instance, a fingernail with horizontal ridges may be a symptom of malnutrition, a pitted nail may reflect psoriasis, or blue nails could indicate circulatory problems. A persistently warm, moist hand can be used with other symptoms to indicate a thyroid disorder. RAYNAUD'S DISEASE, characterized by constricted arteries in the hands, causes chronically cold hands.
arthritis is often apparent in the hand
joints. Unusually long fingers accompanied by an abnormally tall body may suggest MARFAN SYNDROME.The precursor of the human hand may be seen in the paw of the tree SHREW, the lowest
primate. This paw has claws and is capable of only the most rudimentary manual functions. These animals, like squirrels, are tree runners rather than climbers. The hand of the TARSIER is capable of a more advanced degree of grasping ability. It can pick up objects by bending the digits toward the palm; there is a small degree of thumb opposability to other digits, allowing the tarsier to both climb trees and grasp small objects with one hand. Experts once assumed that hominids of the genus Homo, which appeared 1.9 million years ago, were the first to have hands with a true opposable thumb and nails instead of claws, enabling sophisticated hand manipulation. Fossil hand bones recently recovered from a
cave in South
Africa indicate, however, that members of Paranthropus, a group of hominids that appeared about 2 to 2.2 million years ago, also possessed hands capable of precision manipulation. While these early hominids may have been capable of using tools, it is thought that
primate forbearers of humans may have been equipped with a hand of human form before they had developed a brain that could exploit its potential.Roughly 90 percent of all human beings are thought to be right-handed, meaning that the right hand is preferred over the left one in the performance of tasks. (Only a small percentage of people are ambidextrous, and some of them still manifest a clear division of labor between their hands.) This right-side tendency, which extends to the rest of the body, is peculiar to humans. Other animals may exhibit left- or right-sidedness, but the chances of either are essentially equal.This human difference appears to arise from the advanced development of the human brain (see
brain bilateralism). The left hemisphere of the human brain is usually dominant--the reason for this, if any, is unknown--and the
nervous system is so structured that the left hemisphere
Controls the right side of the body. Sometimes the brain is more symmetrical, however, whether through
inheritance or through factors involved in fetal
growth. In such individuals the right hemisphere tends to dominate, so that the person is left-handed. Left-handed and ambidextrous individuals also have a thicker corpus callosum (the fiber bundle linking the left and right hemispheres). As a group, left-handed and ambidextrous individuals seem to exhibit wider ranges of various physical and mental characteristics than do right-handed ones. The strong cultural bias often observed toward right-
handedness is thought to have forced many natural left-handers to use their right hand."
handedness" is another sense is widely observed in nature. For example, in many species of spiral-shelled animals and of spirally climbing
vines, the spiral commonly winds in only one way (as in "right-handed" or "left-handed" screws). For the meaning of
handedness in chemistry and
physics, respectively, see
stereochemistry and SYMMETRY.
bibliography: Bradshaw, John, "The Importance of Being Right (or Left-) Handed, New Scientist, Jan. 24, 1985; Porac, Clare, and Coren, Stanley, Lateral Preferences and Human Behavior (1981); Hegstrom, R. A., and Kondepudi, D.K., "The
handedness of the Universe," Scientific American, January 1990.
