How the bones of the skeletal system work together.
A newborn baby normally has 33 vertebrae making up its backbone; but by the time a person reaches adulthood, the number of individual vertebrae has shrunk to 26.
The explanation is during the growth process, the nine bottom vertebrae fuse naturally into just two. In like fashion, we “lose” some 60 bones as we grow up. Some otherwise perfectly normal adults have “extra” bones or “missing” bones. For example, although the normal number of ribs is 12 pairs, some adults may have 11; others may have 13 pairs.
Even a practicing physician might be hard-pressed to identify each of our 200-plus bones and describe the function. An easier way to gain a general understanding of the various functions, capabilities – and weaknesses, too – of our bones is to visualize the skeletal system as a standing coatrack, say, about six feet high.
Call the central pole the backbone. About ten inches down from its top (the top of your skull) is a horizontal crossbar (your shoulders – collarbones and shoulder blades), approximately a foot-and-a-half across. Sixteen or so inches below the bottom of the top crossbar is another, shorter, crossbar, broader and thicker – the pelvic girdle. The coatrack with its two crossbars is now a crude model of the bones of the head and trunk, collectively called the axial skeleton. Its basic unit is the backbone, to which are attached the skull at the top, then the bones of the shoulder girdle, the ribs, and at the bottom, the bones of the pelvic girdle.
By hanging down (or appending) members from the two ends of the top crossbar, and doing the same at the lower crossbar, we would simulate what is called the appendicular skeleton – arms and hands, legs and feet.