The Skeleton and Bones

Bones look like the deadest part of you. Dry, white, hard, they are what remains when everything else has gone, which is why they stand for death itself. Inside a living body, though, bone is one of the busiest tissues there is. It grows, bleeds, heals, stores minerals, manufactures blood, and tears itself down and rebuilds itself so steadily that you have a new skeleton about every ten years.

The frame

An adult human skeleton has 206 bones. Babies have more, around 270, because many bones start as separate pieces that knit together as a child grows; the skull of a newborn has soft gaps between its plates so the head can squeeze through the birth canal. The skeleton divides into two parts. The axial skeleton, 80 bones, is the central column: skull, spine, ribs and breastbone. The appendicular skeleton, 126 bones, is everything attached to it: shoulders, arms, hands, hips, legs and feet.

The skull is 22 bones fused into a case for the brain and a frame for the face, with only one, the lower jaw, able to move. The spine is a stack of 33 vertebrae, cushioned by discs of cartilage, that carries the body's weight and shields the spinal cord while curving in an S that absorbs shock with every step. Twelve pairs of ribs form a cage that protects the heart and lungs and expands with each breath. The pelvis is a bowl of fused bones that carries the trunk's weight into the legs.

More than half the bones are in the hands and feet: 27 in each hand and 26 in each foot, an arrangement that gives the hand its extraordinary dexterity and the foot its springy arch. The femur, in the thigh, is the longest and strongest bone, able to support about thirty times your body weight. The smallest is the stirrup bone deep in the ear, about three millimetres long, one of three tiny bones that carry sound from the eardrum inward.

What bone is made of

Bone is a composite material, like reinforced concrete. About a third of it is protein, mostly collagen fibres, which give it flexibility and stop it shattering. The rest is mineral, mainly calcium phosphate crystals deposited on those fibres, which gives it hardness. Collagen without mineral is rubbery; mineral without collagen is brittle like chalk. Together they produce a material as strong as some steels at a quarter of the weight.

A long bone such as the femur has a dense hard shell, compact bone, surrounding a lighter honeycomb of struts called spongy bone, aligned along the lines of stress like the girders of a bridge. The hollow shaft is filled with marrow. Bones are not solid because they do not need to be; the design puts strength where the loads are and leaves the rest empty, which is why a skeleton weighs only about 15 percent of body weight.

A living factory

Throughout the bone run blood vessels and nerves, and among the mineral live three kinds of cell. Osteoblasts build new bone, laying down collagen and seeding it with mineral. Osteoclasts dissolve old bone, carving tunnels through it. Osteocytes, trapped in the hardened matrix, sense strain and direct the other two. This constant demolition and rebuilding, called remodelling, replaces about ten percent of the skeleton every year. It lets bone adapt: heavy use, as in a tennis player's racket arm, thickens bone, while disuse, as in astronauts or a limb in a cast, thins it fast.

The marrow inside the bones is the body's blood factory. Red marrow, found in the hips, breastbone, ribs, skull and the ends of the long bones, produces about 500 billion cells a day: red blood cells to carry oxygen, white blood cells to fight infection, platelets to clot wounds. Bone also acts as the body's calcium bank, releasing the mineral into the blood when nerves and muscles need it and storing it when there is a surplus.

Growth and healing

Children's bones grow from plates of cartilage near their ends, which steadily turn into bone while adding new cartilage ahead. Around the late teens the plates themselves harden and growth stops; a doctor can estimate a young person's age from an X-ray by how much of the plate remains.

When a bone breaks, it bleeds and swells like any wound. Within days cartilage and soft bone bridge the gap; over weeks this hardens into a lumpy callus of new bone; over months the callus is remodelled back to the bone's original shape until, often, no trace of the break remains. A child's broken arm heals in about a month, an adult's in two or three. Bone is one of the few tissues in the body that repairs itself perfectly rather than scarring.

Joints

Where bones meet, they are held by ligaments, tough bands of collagen, and separated by smooth cartilage lubricated by fluid, so that they glide with less friction than ice on ice. The ball-and-socket joints of the shoulder and hip allow movement in every direction; the hinges of the knee and elbow bend one way; the gliding joints between the wrist bones allow small slides. The joints between the skull plates do not move at all.

Bones through life

Bone mass peaks in the late twenties and declines slowly after. In some people, especially women after menopause, the loss outpaces replacement and bones become porous and fragile, a condition called osteoporosis that makes hips and wrists break from minor falls. Weight-bearing exercise, calcium and vitamin D, which the skin makes from sunlight and which the gut needs to absorb calcium, help keep bone strong. Bones also record a life: skeletons dug up by archaeologists show healed fractures, diet, disease, age and the work a person did, thousands of years after they died.

Why it matters

Without a skeleton you would be a puddle. Bones hold you upright, let muscles move you, shield your brain, heart and lungs, make your blood and bank your minerals. They are also the one part of you likely to outlast you by centuries, carrying the story of your life to whoever finds them.