The human skeletal system comprises bones and other connective tissue, including cartilage and ligaments. Bones are hard and rigid structures that give shape and posture to the human body. Some bones form protective structures, such as the rib cage and the skull, that help prevent trauma to vital organs and soft tissue. Others, such as the bones in limbs, act as levers to facilitate body movements directed by the skeletal muscles. These muscles are attached to bones via tendons, which transmit the pulling forces from the muscles to the bones enabling movement. Similarly, another type of dense connective tissue, the ligaments, connect one bone to another. They stabilize the position of the bones and also allow movement at the joints. Additionally, such movable joints are cushioned by cartilage, which functions as a shock absorber and reduces friction during movement. Due to its flexibility and tensile strength, cartilage tissue also provides support and shape to some body parts. For example, the costal cartilages allow the forward expansion of the rib cage, and the tracheal rings give shape and support to the trachea.