Skeleton

Skeleton, in vertebrates, framework of bones that supports and protects the soft tissues and organs of the body. It acts as an attachment for the muscles, especially those producing movement, and protects vital organs such as the brain, heart, and lungs. It is also a store of calcium, magnesium, sodium, phosphorous, and proteins, while its bone marrow is the site of red blood corpuscle formation. In the adult human body there are about 206 bones, to which more than 600 muscles are attached.

The skeleton consists of the axial skeleton (the skull, backbone, and ribcage) and the appendicular skeleton (limbs). The function of the axial skeleton is mainly protective. The skull consists of 29 bones, 8 being fused to form the cranium, protecting the brain. The vertebral column, or backbone, consists of 33 small bones (or vertebrae). The upper 25 are joined by ligaments and thick cartilaginous disks, and the lower 9 are fused. It supports the upper body and protects the spinal cord, which runs through it. The ribcage consists of 12 pairs of ribs forming a protective cage around the heart and lungs, and assists in breathing. The appendicular skeleton is primarily concerned with locomotion and consists of the arms and pectoral girdle and the legs and pelvic girdle. The limbs articulate with their girdles in ball-and-socket joints that permit the shoulder and hip great freedom of movement but that are prone to dislocation. In contrast, the elbows and knees are hinge joints that permit movement in one plane only but that are very strong.