Bones are major components of the skeletal systems of 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. Bones are also a vital part of the body's physiology. They store calcium, which is essential for nerve and muscle cells. Also, the soft core of bone, the bone marrow, is where red blood cells, certain white blood cells, and blood platelets form.