<P> The Latin word for bone is os, hence the many terms that use it as a prefix--such as osseous and osteopathy . </P> <P> Bone is not uniformly solid, but includes a tough matrix . This matrix makes up about 30% of the bone and the other 70% is of salts that give strength to it . The matrix is made up of between 90 and 95% collagen fibers, and the remainder is ground substance . The primary tissue of bone, bone tissue (osseous tissue), is relatively hard and lightweight . Its matrix is mostly made up of a composite material incorporating the inorganic mineral calcium phosphate in the chemical arrangement termed calcium hydroxylapatite (this is the bone mineral that gives bones their rigidity) and collagen, an elastic protein which improves fracture resistance . The collagen of bone is known as ossein . Bone is formed by the hardening of this matrix around entrapped cells . When these cells become entrapped from osteoblasts they become osteocytes . </P> <P> The hard outer layer of bones is composed of cortical bone also called compact bone . Cortical referring to the outer (cortex) layer . The hard outer layer gives bone its smooth, white, and solid appearance, and accounts for 80% of the total bone mass of an adult human skeleton . Cortical bone consists of multiple microscopic columns, each called an osteon . Each column is multiple layers of osteoblasts and osteocytes around a central canal called the haversian canal . Volkmann's canals at right angles connect the osteons together . The columns are metabolically active, and as bone is reabsorbed and created the nature and location of the cells within the osteon will change . Cortical bone is covered by a periosteum on its outer surface, and an endosteum on its inner surface . The endosteum is the boundary between the cortical bone and the cancellous bone . </P> <P> Cancellous bone also known as trabecular or spongy bone tissue is the internal tissue of the skeletal bone and is an open cell porous network . Thin formations of osteoblasts covered in endosteum create an irregular network of spaces, known as trabeculae . Within these spaces are bone marrow and hematopoietic stem cells that give rise to platelets, red blood cells and white blood cells . Trabecular marrow is composed of a network of rod - and plate - like elements that make the overall organ lighter and allow room for blood vessels and marrow . Trabecular bone accounts for the remaining 20% of total bone mass but has nearly ten times the surface area of compact bone . </P>

What is the dense bone on the outside surfaces of a skeletal element called