Hand

• 1. Introduction

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  An adult human male's hand weighs about a pound, yet it carries some of the densest areas of nerve endings in the entire body. The fingers are the richest source of tactile feedback we possess, and they hold the greatest positioning capability of any part of us. Twenty-seven bones sit packed inside this small space. The thumb stands apart from the rest, parallel to the arm, able to swing opposite the fingers in a motion called opposition. That single trick separates a true grasping hand from a paw, a claw, or a talon. Why do koalas have fingerprints almost identical to ours, while raccoons are said to have hands despite lacking opposable thumbs? How did a structure built from a fish's fin become the working organ of human consciousness? And why does the side of your hand stay lighter than the back, even in dark-skinned people? The answers begin with what counts as a hand at all, and they end deep in the evolution of the brain.

• 2. What Counts As A Hand

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  Only the mammalian order of primates has what science calls a true grasping hand. Humans, chimpanzees, monkeys, and lemurs all carry one at the distal end of each arm. Many other animals own grasping appendages of similar form, but paws, claws, and talons do not qualify. Calling the front paw of an animal a hand is an example of anthropomorphism, a way of reading human shape into other bodies. The hard rule is the opposable thumb, the ability to bring the thumb opposite the fingers. A few outliers blur the edges. The koala has two opposable thumbs on each hand and fingerprints extremely similar to human ones, so it is often described as having hands rather than paws. The raccoon is usually granted hands too, even though it lacks opposable thumbs entirely. Apes and monkeys are sometimes said to have four hands, because their toes are long and the hallux, the big toe, is opposable enough to act like a thumb. That lets the feet work as hands. Evolutionary anatomists stretch the word even further, using it to compare the three digits of the bird hand with the dinosaur hand when they research whether the same two digits were lost in each.

• 3. Twenty-Seven Bones

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  Eight short carpal bones form the wrist, arranged in two rows. The proximal row, the scaphoid, lunate, triquetral, and pisiform, meets the bones of the forearm. The distal row, the trapezium, trapezoid, capitate, and hamate, meets the bases of the five metacarpal bones. Each metacarpal head then joins the proximal phalanx of a finger or thumb. Those joints where the digits meet the palm are the metacarpophalangeal joints, known to everyone as the knuckles. Fourteen phalanges make up the digits, numbered I to V from thumb to little finger when the palm faces up. Each of the four fingers carries three phalanx bones, proximal, middle, and distal. The thumb, connected to the first metacarpal bone and the trapezium, has only a proximal and a distal. Small, almost spherical sesamoid bones hide in tendons at the palmar aspect of the first metacarpophalangeal joints, and their count varies from person to person. A pair sits at virtually every thumb metacarpophalangeal joint, but the rest are a matter of odds: the interphalangeal joint of the thumb in 72.9 percent of people, the little finger's metacarpophalangeal joint in 82.5 percent, and the index finger's in 48 percent. Because the forearm rotates in supination and pronation, anatomists sometimes fold the ulna and radius into the skeleton of the hand.

• 4. Arches And The Master Digit

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  The carpal bones bend into two transversal rows, each forming an arch that is concave on the palm side. The proximal arch must adapt at once to the radius and to the distal carpal row, so it stays flexible. The distal arch is rigid, anchored by the capitate, which acts as its keystone and moves with the metacarpal bones. Ligaments, not the shapes of the bones, hold these arches together, which is why the wrist is more stable in flexion than in extension. The transverse carpal ligament and the intercarpal ligaments also build the carpal tunnel. The longitudinal arches, the rays running through each finger and its metacarpal, differ in mobility. The thumb's ray is the most mobile and least longitudinal. The little finger keeps some movement, and the remaining rays are firmly rigid, though the index finger gains independence from the arrangement of its flexor and extension tendons. The middle finger metacarpal is the most rigid of all. Above these sit four oblique arches between thumb and fingers. The thumb is the master digit of the hand, the one that gives value to all the others. With the index and middle finger it forms a dynamic tridactyl configuration that handles most grips not needing force. The ring and little fingers stay more static, a reserve that joins the palm when great force is needed, with the little finger's arch supplying a locking mechanism for power grip.

• 5. Muscles And Wiring

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  The muscles that move the hand split into extrinsic and intrinsic groups. The extrinsic ones are the long flexors and extensors, named for muscle bellies that sit out on the forearm. Two long flexors run along the underside of the forearm and reach the fingers by tendons. The deep flexor attaches to the distal phalanx and the superficial flexor to the middle phalanx, and together they bend the fingers. The extensors live on the back of the forearm, straighten the digits, and travel within six separate compartments. The index finger and little finger each get an extra extensor used for pointing, while the thumb's two extensor tendons frame the anatomical snuff box. Intrinsic muscles stay inside the hand: the thenar group of the thumb, the hypothenar group of the little finger, the interosseous muscles between the metacarpals, and the lumbrical muscles, which are unusual because they arise from the deep flexor and have no bony origin. Three nerves run the whole apparatus. The radial nerve drives the finger extensors and the thumb abductor. The median nerve handles the flexors of the wrist and digits plus the thumb's opponens. The ulnar nerve takes the remaining intrinsic muscles. Sensation divides along similar lines, though the pattern varies widely from person to person, holding steady only for the little finger and the volar surface of the index finger.

• 6. Skin Blood And Touch

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  The glabrous, hairless skin of the palm is thick and bends along the hand's flexure lines, where it binds tightly to the tissue and bones beneath. On dark-skinned people especially, the palm runs much lighter than the back of the hand. Genes expressed in the dermis of palmoplantar skin inhibit melanin and the ability to tan, while thickening the stratum lucidum and stratum corneum of the epidermis. Papillary ridges, the fingerprints, cover every grasping surface and act as friction pads. The dorsal skin is the opposite: thin, soft, pliable, able to be moved across the hand up to 3 centimeters, an important input to the cutaneous mechanoreceptors. Between the digits sit the webs of the hand, folds of skin also called interdigital folds. Blood arrives through the ulnar artery and the radial artery, which build three arches, the dorsal carpal arch, the deep palmar arch, and the superficial palmar arch. Those arches and their anastomoses feed the palm, fingers, and thumb. Deoxygenated blood drains through the dorsal venous network and leaves by the cephalic vein and the basilic vein. One quiet marker of all this development shows up in proportion: the ratio of index finger length to ring finger length is shaped by the embryo's exposure to male sex hormones in utero, sitting below 1 in both sexes but lower in males on average.

• 7. When The Hand Breaks Down

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  A scaphoid fracture, a break of one of the carpal bones, is the commonest carpal fracture and can be slow to heal because so little blood reaches the bone. Other breaks have their own names. The base of the thumb gives rise to Rolando fractures, Bennet's fracture, and Gamekeeper's thumb, while a break to the neck of a metacarpal is a Boxer's fracture. Genetic disorders reshape the hand from birth. Polydactyly adds extra fingers, and one cause is Catel-Manzke syndrome. Syndactyly fuses the fingers together. Ectrodactyly removes one or more central fingers, and amelia leaves a person born without one or both hands. Hereditary multiple exostoses of the forearm, also called hereditary multiple osteochondromas, deforms the hand and forearm in children and adults. Disease arrives later for others. Rheumatoid arthritis, an autoimmune condition, attacks the joints of the fingers. Some problems answer to hand surgery, including carpal tunnel syndrome, a painful compression of the median nerve, and Dupuytren's contracture, in which the fingers bend toward the palm and cannot be straightened. Injury to the ulnar nerve can leave some fingers unable to flex at all, a reminder of how closely the nerve supply and the moving hand are bound together.

• 8. From Fin To Consciousness

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  The precursors of the intrinsic muscles of the hand are already present in the earliest fishes. The hand evolved from the pectoral fin, which makes it far older than the arm in evolutionary terms. The prehensile hands and feet of primates grew from the mobile hands of semi-arboreal tree shrews, alongside changes in the brain and the move of the eyes to the front of the face, which gave the stereoscopic vision needed for controlled grasping. When the great apes, the Hominidae, took up an erect bipedal posture, the hands were freed from locomotion, opening the way to the precision and range of motion humans now have. Anatomy alone cannot explain human dexterity. Primates evolved direct, monosynaptic connections between neurons in cortical motor areas and the spinal motoneurons of the hand muscles, placing the hands closer to the brain. The recent evolution of the human hand is therefore a direct result of the development of the central nervous system, making the hand a direct tool of consciousness. Primitive traits still linger: pentadactyly, the hairless palm, and the os centrale found in human embryos, prosimians, and apes. Human hand proportions are plesiomorphic, with elongated thumbs and short hands closer to Miocene apes than to living primates. The thumbs of Ardipithecus ramidus are almost as robust as ours, and features absent in modern humans survive in Australopithecus, Paranthropus, and Homo floresiensis. One disputed idea even holds that the hand's form suits a compact fist for fighting, though most researchers credit tool use and production as the far stronger force shaping the hand we carry today.