The skeletal system is comprised of bones, cartilage, joints and ligaments. Though mostly bone, the entire system comprises approximately 20 percent of a human’s body mass. The major components of the skeleton are divided between the axial skeleton and the appendicular skeleton. The parts of the axial skeleton support the head, neck, and trunk and protect the brain, the spinal column and organs. The appendicular skeleton is comprised of the limbs and their supporting girdles, which allow for functional movement.
The Skull (Axial)
The skull is the most complex bony structure in the body made up of 22 cranial and facial bones. Cranial bones protect the brain and provide attachments sites for head and neck muscles. Facial bones create the structure of the face, provide cavities for sensory organs, anchor the teeth, and provide attachments for muscles used in speech and facial expression.
The Vertebral Column (Axial)
The vertebral column consists of 31 separate vertebrae. The vertebrae are divided into seven cervical vertebrae at the neck, twelve thoracic vertebrae parallel to the chest, five lumbar vertebrae at the lower back, five fused vertebrae at the sacrum and two fused vertebrae at the coccyx. The vertebral column encases and protects the spinal cord as well as supporting a deep network of back muscles that allow for trunk movement, breathing and movement of upper body limbs.
Bony Thorax (Axial)
Also known as the thoracic cage, this includes the thoracic vertebrae, sternum, ribs and costal cartilage. The rib cage consists of twelve ribs held together by the sternum. Ribs 1 through 7 attach directly to the sternum, ribs 8 through 10 attach indirectly through costal cartilage and ribs 11 and 12 are floating ribs that do not attach to the sternum at all. The bony thorax serves as a support for the shoulder girdle as well as protecting the vital organs. It also supports muscles key to breathing.
Shoulder and Pelvic Girdle (Appendicular)
The shoulder girdle includes the collarbone and shoulder blades. These two combine to form the sockets and support for the shoulder joints. The collarbones also brace the arms in position away from the bony thorax and hold the shoulder blades in place. The pelvic girdle is comprised of two hip bones that join at the sacrum to create a bowl-like structure that supports the lower organs. The lower limbs attach into the pelvic girdle to support the weight of the upper body.
Upper and Lower Limbs (Appendicular)
The upper limbs consist of the bones that create the arm, forearm and hand. The upper limbs support muscular structure that allows for arm movement, elbow flexion and articulation of the fingers that allow for everyday motions like grasping, reaching, lifting, and carrying. Lower limb structure includes the bones of the thigh, lower leg, and the foot. The lower limbs serve as attachment sites for the substantial muscles of the leg. The combination of bones and muscles enable the lower limbs to support the entire weight of the upper body as well as allow motions such as sitting, walking and running.