The Human Skeletal System: An Overview

Movement of the human body as a whole is possible only because the contractile force generated by muscles is applied to a rigid, jointed internal framework -- the skeletal system -- that the muscles can pull against. The adult human skeleton is made up of 206 bones (along with cartilage, ligaments and ancillary connective tissue), and for convenience of description it is divided into two major divisions: the axial skeleton, which forms the main vertical axis of the body (the skull, vertebral column, sternum and ribs), and the appendicular skeleton, which forms the two pairs of limbs together with the girdles that attach them to the axial skeleton. Detailed bone-by-bone counts for each division are given in sections 18.6 and 18.7.

Besides making muscular movement possible by providing rigid levers and points of muscle attachment, the skeletal system performs several other essential functions. It gives the body its definite shape and provides overall mechanical support, holding the softer tissues and organs in their proper place. It protects delicate internal organs from mechanical injury -- most notably, the skull encloses and protects the brain, the vertebral column encloses and protects the spinal cord, and the rib cage encloses and protects the heart and lungs. Certain bones, particularly the flat bones and the ends of long bones, contain red bone marrow, which is the site of haemopoiesis, the continuous formation of new red blood cells, white blood cells and platelets throughout life. The skeleton is also …