Bone, rigid body tissue consisting of cells embedded in an abundant hard intercellular material. The two principal components of this material, collagen and calcium phosphate, distinguish bone from such other hard tissues as chitin, enamel, and shell. Bone tissue makes up the individual bones of the human skeletal system and the skeletons of other vertebrates. The functions of bone include :-structural support for the mechanical action of soft tissues, such as the contraction of muscles and the expansion of lungs,  protection of soft organs and tissues, as by the skull, provision of a protective site for specialized tissues such as the blood-forming system (bone marrow), and a mineral reservoir, whereby the endocrine system regulates the level of calcium and phosphate in the circulating body fluids.