Muscles are on the outside of the endoskeleton. Cartilage and bone are types of connective tissue . Image from W.H. Freeman and Sinauer Associates, used by permission. The skeleton and muscles function together as the musculoskeletal system. Certain cells in the bones produce immune cells as well as important cellular components of the blood.
Bone also helps regulate blood calcium levels, serving as a calcium sink.
Rapid muscular contraction is important in generating internal heat, another homeostatic function. The axial skeleton consists of the skull, vertebral column, and rib cage. The human skull, or cranium , has a number of individual bones tightly fitted together at immovable joints . The vertebral column has 33 individual vertebrae separated from each other by a cartilage disk. These disks allow a certain flexibility to the spinal column, although the disks deteriorate with age, producing back pain.