It is composed of
organic materials. The chiefly organic constituent of bone is needle-like crystals of hydroxyapatite, a form of calcium phosphate. Also present in variable amount are sodium, magnesium, potassium, chloride, fluoride, hydrogen carbonate and nitrate ions. The mineral part of the matrix is resistant to tension thereby giving it the ability to withstand compression, strains, falling upon it and to resist tension.
The bone provides support, protection and some metabolic functions. The reabsorption and construction process of bone enable it to meet any change in mechanical requirement of the animal during development. Numerous small openings penetrates the surface of the bone, through which nerves and blood vessels cross into the bony tissue and marrow.