The structural units are osteons, which are elongated cylinders acting as weight-bearing pillars that can withstand high levels of mechanical stress. The osteons' 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. The inner layer consists mainly of osteoblasts, cells that are constantly renewed in the bone.
Osteoblasts make and secrete collagen, giving bone a measure of elasticity.
Also, 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 major components of skeletal systems in nearly all vertebrate animals. There are 206 bones in the adult human, which make up 14 percent of total body weight. Bone is composed of both living and nonliving materials and provides a framework for the body and a surface for the attachment of muscles. The skull and rib bones protect the brain and heart.