In vertebrates, cervical vertebrae (singular: vertebra), also called the cervical spine, are those vertebrae immediately inferior to the skull. The cervical vertebrae, more commonly, the neck, is a structure of the skeletal system of the body composed of seven individual, irregularly shaped bones. That these circular bones sit one on top of the other instead of being fused together as one solid structure allows for good flexibility so that a person can turn his or her neck to either side without causing any damage to the neck. Being stacked on top of one another, they form a protective cavity, down through which the most important part of the spinal cord extends. In many species, though not in mammals, the cervical vertebrae bear ribs. In many other groups, such as lizards and saurischian dinosaurs, the cervical ribs are large; in birds, they are small and completely fused to the vertebrae. The transverse processes of mammals are homologous to the cervical ribs of other amniotes. In humans, cervical vertebrae are the smallest of the true vertebrae, and can be readily distinguished from those of the thoracic or lumbar regions by the presence of a foramen (hole) in each transverse process, through which passes the vertebral artery.