The skeleton is subdivided into two major divisions—the axial skeleton and the appendicular skeleton. The axial skeleton forms the vertical, central axis of the body. It includes all of the bones of the head, neck, chest, and back. It protects the brain, spinal cord, heart, and lungs. It also serves as the attachment site for muscles that move the head, neck, and back and for muscles that act across the shoulder and hip joints to move their corresponding limbs.
The axial skeleton of the adult consists of 80 bones, including the skull, the vertebral column, and the thoracic cage. The skull is formed by 22 bones. Also associated with the head are seven additional bones, including the hyoid bone and the ear ossicles (three small bones found in each middle ear). The vertebral column consists of 24 bones, each called a vertebra, plus the sacrum and coccyx. The thoracic cage includes the 12 pairs of ribs, and the sternum, the flattened bone of the anterior chest.
The skull is the skeletal structure of the head that supports the face and protects the brain. The facial bones underlie the facial structures, form the nasal cavity, enclose the eyeballs, and support the teeth of the upper and lower jaws. The rounded cranium surrounds and protects the brain and houses the middle and inner ear structures. In the adult, the skull consists of 22 individual bones, 21 of which are immobile and united into a single unit. The 22nd bone is the mandible (lower jaw), the skull's only moveable bone.
The vertebral column is also known as the spinal column or spine. It consists of a sequence of vertebrae (singular = vertebra), each separated and united by an intervertebral disc. Together, the vertebrae and intervertebral discs form the vertebral column. It is a flexible column that supports the head, neck, and body and allows for their movements. It also protects the spinal cord, which passes down the back through openings in the vertebrae. The vertebral column originally develops as a series of 33 vertebrae, but this number is eventually reduced to 24 vertebrae, plus the sacrum and coccyx. The vertebral column is subdivided into five regions, with the vertebrae in each area named for that region and numbered in ascending order from the top down.
The thoracic cage (rib cage) forms the thorax (chest) portion of the body. It consists of 12 pairs of ribs with their costal cartilages and the sternum. The ribs are anchored posteriorly to the 12 thoracic vertebrae.
The middle ear consists of a space spanned by three small bones called the ossicles. The three ossicles are the malleus, incus, and stapes, which are Latin names that roughly translate to hammer, anvil, and stirrup. The single suspended U-shaped hyoid bone, though not directly connected to the skull, lies below the mandible in front of the neck. This horseshoe-shaped bone is the site of attachment for the muscles of the jaw, larynx and tongue.
The appendicular skeleton includes all bones of the upper and lower limbs, plus the bones that attach each limb to the axial skeleton. There are 126 bones in the appendicular skeleton of an adult.
These bones are divided into two groups: the bones of the limbs themselves and the girdle bones that attach the limbs to the axial skeleton. The bones of the shoulder region form the pectoral girdle, which anchors the upper limb to the thoracic cage of the axial skeleton. The lower limb is attached to the vertebral column by the pelvic girdle.
This text is adapted from Openstax, Anatomy and Physiology 2e, Section 7: Axial Skeleton , Openstax, Anatomy and Physiology 2e, Section 14.1: Sensory perceptions and Openstax, Anatomy and Physiology 2e, Section 8: The Appendicular Skeleton
