The adult human skeleton is a bony framework that supports the body and gives it shape. It also protects the internal organs and anchors the body’s muscles. The skeleton is composed of 206 bones and is divided into two parts. The axial skeleton – the skull, spine, and rib cage – consists of 80 bones and protects the brain, spinal cord, heart, and lungs. The appendicular skeleton has 126 bones and consists of the bones of the limbs, the collarbones, the shoulder blades, and the bones of the pelvis.
View of the skull from below
The skull has several holes through which vital structures connect with the brain. The spinal cord passes through the foramen magnum, the largest hole. The carotid arteries pass through smaller, paired holes to supply blood to the brain.
Structure of a long bone
A long bone, such as the femur, has a marrow-filled cavity surrounded by spongy bone. The next layer is denser, compact bone. Covering the outer surface is a membrane (periosteum), which contains nerves and a network of blood vessels.
Mature bone cell
This highly magnified view shows a mature bone cell called an osteocyte. Such cells are found in compact bone, trapped in spaces called lacunae. Osteocytes are responsible for laying down calcium to maintain bone strength.