skeleton, in anatomy, the stiff supportive framework of the body. The two basic types of skeleton found among animals are the exoskeleton and the endoskeleton. The shell of the clam is an exoskeleton composed primarily of calcium carbonate. It provides formidable protection, but it is bulky and severely restrictive of movement. The smallest exoskeletons are found on microscopic animals such as diatoms and certain protozoans. Coral reefs are made up of the accumulated exoskeletons of the coral polyp. The firm, flexible, chitinous (horny) insect skeleton is a combination of protective armor and a framework for attachment of the muscles used in rapid movement. The disadvantage of an exoskeleton is that it is nonliving, and must be shed periodically to allow for growth—a process limiting the maximum size of the organism.
The endoskeleton, a framework of living material enclosed within the body, permits larger size coupled with freedom of movement and is characteristic of vertebrate animals. In certain fish, it is made up entirely of cartilage, but in most vertebrates it is a mixture of bone and cartilage. The general arrangement of skeletal parts into skull, spinal column, ribs, and appendages is the same in all vertebrates. In addition to its supportive function, the skeleton provides sites for the attachment of the muscles used in movement and shields vital organs such as the brain and lungs. The skeleton of birds is especially adapted for flight; the bones are modified into light, hollow tubes penetrated by air sacs.
The human skeleton consists of 206 bones held together by flexible tissue consisting of cartilage and ligaments. It is composed of two basic parts, the axial and the appendicular skeletons. The axial skeleton includes the cranium, jawbone, ribs, sternum, and spinal column. The appendicular skeleton is made up of the upper (shoulder or pectoral) and lower (pelvic) girdles (see pelvis) and the bones of the arms and legs. Many diseases associated with the skeleton occur at the joints, notably the various types of arthritis, although such diseases as bone cancer may directly affect the skeleton. Skeletal remains are vital to physical anthropologists, who use them to trace human evolution.
See P. Shipman, A. Walker, and D. Bichell, The Human Skeleton (1985).