There are two basic structural types of bone in mammals, compact and spongy. Compact bone is very dense and hard on the outside, and makes up most of the bones in the arms and legs of humans.

View a second image of human compact bone.

The structural units of human compact bone are osteons, which are elongated cylinders. Acting as weight-bearing pillars, osteons can withstand high levels of mechanical stress. At the osteon's center is a hollow canal that acts as a central passageway for blood vessels and nerves. A thin membrane, termed the periosteum, surrounds compact bone. The outer layer of this membrane contains blood vessels and nerves that travel into the bone, while the inner layer consists mainly of osteoblasts. These are cells that are constantly renewed in the bone, and which produce and secrete collagen, giving bone a measure of elasticity. In addition, osteoblasts secrete mineral salts formed from calcium and phosphorus, which prevent the bones from breaking easily, as well as rebuilding bone if necessary.

Bones are the major components of skeletal systems in nearly every vertebrate animal. There are 206 bones in the adult human, which make up 14 percent of the total body weight. Bone is composed of both living and nonliving materials. By providing a framework for the body and a surface for the attachment of muscles, the compact bones allow humans to walk upright. Protection for vital organs is provided by other bones, as exemplified by the skull and rib bones, which protect the brain and the heart. Bones also play a major role in human physiology by providing storage for the calcium that is essential for functioning nerve and muscle cells. Additionally, the soft core of bone, the bone marrow, serves as the site for formation of red blood cells, certain white cells, and blood platelets.

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