It consists of four parts : Periosteum, matrix, endosteum and bone marrow.
(a) Periosteum : It is a thick and tough sheath that
forms an envelop around the bone. It is composed j of collagen (= white) fibrous tissue. Bundles of iperiosteal collagen fibres, penetrate the bone matrix to provide a firm connection between the two. The ' periosteum also contain bone-forming cells, the
osteoblasts which produce new bone material.
(b) Matrix : It is composed of a protein called ossein. :
The main salts found in the matrix are calcium phosphate, calcium carbonate, sodium chloride and magnesium phosphate. The haversian canals are present in the matrix. Each haversian canal contains an artery, a vein, a lymph vessel, a nerve and some bone cells, all packed in with connective tissue. The matrix has numerous inactive bone cells, the osteocytes. An osteocyte is surrounded by a j fluid-filled space, the bone lacuna, which leads into fine radiating channels, the canaliculi. The matrix of the bone occurs as layers called lamellae. )
(c) Endosteum: It is present outer to the bone marrow cavity. It comprises white fibrous tissue and
osteoblasts, the long bone thus grows in thickness from two sides. This type of growth is called bidirectional growth.
(d) Bone marrow : In long bones such a limb bones (humerus, femer, etc.) a cavity called bone marrow cavity is present inner to the endosteum. The bone marrow cavity is filled with a soft and semi-solid fatty neuro-vascular tissue termed as bone marrow