Bone
Bone is a type of skeletal tissue. Bones are also organs made largely of this tissue and surrounded by a membrane called the periosteum. This study of bones is called osteology. Bone tissue contains living cells embedded in a hard matrix. This matrix consists mostly of calcium phosphate (as hydroxylapatite) and other calcium minerals held together by collagen (a protein) and other organic substances. The major types of bone tissue exist. Compact bone tissue has many rodlike Haversian systems, each containing many concentric cylindrical layers (lamellae) surrounding a central blood vessel . Between these layers the bone cells (osteocytes) lie within spaces called acunae. Tiny pores (canalculi) connect the osteocytes with one another and with the Blood vessel, allowing oxygen and nutrients to reach the cells. Cancellous, or spongy bone tissue has a bone network of rigid beams (trabeculae). The spaces between these trabeculae contain marrow, a blood forming tissue. Young marrow is