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Bone (Physiology a hard dense opaque)

in physiology, a hard, dense, opaque substance used as the internal framework of man, the vertebrata and some cephalopoda, and as the external covering of several classes of animals. It is composed partly of an organic (or animal) and partly of an inorganic (or material is a trifle under half the weight earthy) material. In a child the of the bone, in an adult four-fifths, and in an old person seven-eights. The animal part of bone consists of cartilage, with vessels, medullary membrane and fat. Three hours' boiling will convert it into gelatine. The earthy part consists of phosphate and carbonate of lime, with smaller portions of phosphate and carbonate of magnesia. The outer portion of a bone is in general compact and BONESET strong, the interior reticular, spongy, or cancelled, that is, having spaces or cells (called cancelli) communicating freely with each other. The hard surface of bone is covered by a firm, tough membrane called the periosteum. In the compact tissue are vascular canals called havenian canals. There are in bone pores coalescing into a lacuna beneath. It has blood vessels and nerves. Bones may be classified into long, short, flat, and irregular. A long bone is divided into a shaft and two extremities. There are 198 bones in the fully developed human skeleton.