Born with more bones than you have now
An adult SKELETON usually has 206 bones, but a newborn baby has around 300. Many start out as separate pieces, partly made of bendy CARTILAGE, and fuse together as a child grows. The bones of the SKULL, or CRANIUM, are loose at birth, with soft gaps between them that let a baby's head squeeze through at birth and give the brain room to grow; they knit together in the first couple of years. A baby's KNEECAP is not bone at all to begin with, but cartilage that hardens over the first few years.
More than half of all your bones are in your hands and feet. The longest and strongest is the FEMUR, the thigh bone, roughly a quarter of your height. The smallest is the stirrup in your ear, about three millimetres long.
Bone is alive
A dried skeleton in a science classroom looks like a lifeless frame, but living BONE is busy. It is constantly being broken down and rebuilt, so over about ten years most of your skeleton is replaced. It also gets stronger where it is used: a tennis player's racket arm has noticeably thicker bones than the other arm. Astronauts floating in space, where their bones carry no weight, can lose one or two per cent of their bone mass each month, and they exercise for hours a day on the space station to slow it down.
Bones store about 99 per cent of the body's CALCIUM, and the soft MARROW inside the larger ones makes blood cells. The SPINE, or BACKBONE, is a stack of thirty-three VERTEBRAE, some of them fused, protecting the spinal cord. The only bone not joined to any other is the small hyoid bone in your throat, which anchors the tongue.
A broken bone mends itself. Within days the body builds a lump of rough new bone, called a callus, bridging the break, and over the following months it is slowly reshaped until the bone is almost as good as before. A plaster cast does not heal the bone; it simply holds the two ends still while the bone does the work.
Cracking knuckles and broken collarbones
The popping sound when someone cracks a KNUCKLE comes from the fluid inside the JOINT. Pulling the joint apart lowers the pressure, and a bubble of gas forms suddenly. An American doctor, Donald Unger, cracked the knuckles of his left hand, but not his right, at least twice a day for about sixty years to test whether it caused arthritis. Neither hand developed it, and his study won him a spoof science prize in 2009.
The COLLARBONE is one of the most commonly broken bones, usually in a fall onto an outstretched arm, because it connects the SHOULDER to the STERNUM, the breastbone, and takes the jolt. The RIBS curve round from the spine to form the RIBCAGE, protecting the heart and lungs, and the JAW is the only bone in the skull that moves. Other animals build their frames very differently, as our safari animals page shows with the giraffe's neck. The PELVIS, the TIBIA and the rest of the frame are moved by the muscles described on our human body page.