You are born with 300 bones, today 206

At birth you carry around 300 bones, following the classic count by the German anatomist August Rauber in the early twentieth century. The average adult has about 206, according to Gray's Anatomy. The difference isn't that any go missing: it's cartilage that hardens into bone and blocks that fuse together. Four big fusions account for most of the drop.

1. The frontal. You're born with two, one on each side. The metopic suture closes between two and eight years old in most children.

2. The occipital. Four separate plates at birth. The posterior fontanelle closes at three months and the sutures finish sealing before age three.

3. The sacrum. Five separate vertebrae in childhood. They fuse from top to bottom through adolescence; in many people the block isn't finished until after thirty.

4. The coccyx. Three to five tiny vertebrae at birth. They end up as a single bone in most adults.

Two hundred and six is an average: some adults have 205, some 208. The count depends on the odd lumbar rib and on the sesamoid bones each person happens to grow in hands and feet. Nobody loses bones: they're just counted differently.