In the first stages of a baby's development there is no bone. In its place is cartilage, or gristle. The bone begins to form in the cartilage at growth centers according to a definite scheme. Long bones, as for instance the thigh bone and the shinbone (femur and tibia), consist at this stage of the shaft which is called the diaphysis and a growing center at each end called an epiphysis. These epiphyses grow so much on schedule that the age of a child may be pretty accurately determined by X-ray.
Yet bone does not grow simply by adding on. It is continuously being reconstructed. Cells called osteoclasts tear down bone and osteoblasts rebuild it with new. As one grows old and uses the bones less, the osteoclasts are apt to get the upper hand. This is best shown in the old man's chin after the loss of his teeth when much of the bone of the jaw is absorbed, causing an upward curve.
There are channels for blood and much soft tissue in the hard bone which is mostly calcium, as is an oyster shell. If you burn a bone, its shape remains but it becomes brittle like an egg shell. If you immerse it in acid, the calcium and other minerals disappear and it is now much like a puppy's rubber bone.
The finished product is an excellent piece of architecture and engineering. One practically never sees an absolutely straight bone. It is curved to fit its function. It is enlarged where a bearing surface is needed, or rounded to fit into another bone at a joint. It has ridges, knobs, and rough places for the attachment of ligaments and tendons. It is dense where strength is needed and spongy where weight must be saved. To the knowing eye an old bone is a wonderful object.
One skilled in osteology can tell a female skeleton from a male. The female is usually smaller and the bones are lighter. The bony thorax, or chest, is narrower. The shoulders are narrow and the hips broad. This latter feature is so because the one chief function of the sex is to bear the future members of the race, and the broad pelvis makes room for the child. Before the days of frequent Caesarian section we said that the angle of the pelvic bones in front was widened in the female as all humanity had to pass under the arch. Incidentally, the broad hips result in the thigh bones being well apart at the top and coming together below. Hence all women are knock-kneed. It was said of a drama critic that he knocked everything in the show except the chorus girls' knees and God had anticipated him there.
He, who would take care of broken bones, should have knowledge of anatomy, physiology, and likewise mechanical engineering, not to mention old-fashioned plumbing. Nature heals here much as a plumber of the last century "wiped a joint" in a lead pipe. With the two ends of bone together, "callus" or new bone is spread around the juncture as molten lead was allowed to cool around the pipe. If the ends touch each other firmly and evenly, the less callus is necessary and the quicker the union occurs. When the extra callus is later absorbed, a perfect union may result and no trace of the injury remains.
But physiology takes precedence over plumbing here. If the ends are displaced to the side so that they barely meet, nature is still able to hold them together with callus and later smooth off the corners. Right here the surgeon must not forget mechanical engineering. The bones are so designed that the muscles and tendons pull in just the right lines of force. If the ends meet squarely but are much angulated at the site of fracture, then the muscles have to pull in the wrong direction and do not work well, thus disabling the victim.
Possibly the most striking example occurs with a Colles fracture, which is a break in the radius, one of the two bones of the forearm, just above the wrist joint. If the lower fragment is left tipping backwards only ten degrees, the result may be a poor wrist thereafter.