These bones are expanded into broad, flat plates, as in the cranium (skull), the ilium (pelvis), sternum and the rib cage. The flat bones are: the occipital, parietal, frontal, nasal, lacrimal, vomer, hip bone (coxal bone), sternum, ribs, and scapulae. ... In an adult, most red blood cells are formed in flat bones.
If you look at Cosmos episode 1, I'm sure you find an answer there.
