The basic chemical in bone, which gives bone its hardness and strength, is calcium phosphate.ĭevelopment. Bone tissue also contains a large number of nerves. Blood vessels course through every layer of bone, carrying nutritive elements, oxygen, and other products. The innermost portion of the bone is a hollow cavity containing marrow. Encased within these layers is the tissue that makes up most of the volume of bone, called cancellous or spongy bone because it contains little hollows like those of a sponge. Its composition is fibrous rather than solid and it gives bone its resiliency. Beneath the periosteum lie the dense, hard layers of bone tissue called compact bone. This membrane encloses all bones completely except at the joints where there is a layer of cartilage. It gives support to the tendons that secure the muscle to the bone and also serves as a protective sheath. The outermost layer, the periosteum, is a thin, tough membrane of fibrous tissue. Bone is not uniform in structure but is composed of several layers of different materials. The bones themselves act as a storehouse of calcium, which must be maintained at a certain level in the blood for the body's normal chemical functioning. The bone marrow in the center manufactures blood cells. Other bones, such as the jaw and the bones of the fingers, are concerned chiefly with movement. Some, such as the pelvis, are mainly supporting structures. An example is the skull, which encloses the brain, the back of the eyeball, and the inner ear. Some bones have a chiefly protective function. Bones, ligaments, muscles, and tendons are the tissues of the body responsible for supporting and moving the body.

Collectively they form the skeletal system, a structure bound together by ligaments at the joints and set in motion by the muscles, which are secured to the bones by means of tendons. There are 206 separate bones in the human body. See anatomic Table of Bones in the Appendices for regional and alphabetical listings of bones, and see color plates 1 and 2. any distinct piece of the skeleton of the body.

Define flat bones