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.

skeleton, in winter sports, a type of small, very low, steel-frame sled on which one person, lying face down, slides headfirst down snowy hillsides or down steeply banked, curving, iced chutes similar to those used in luge and bobsledding. Steering is accomplished by shifting weight or dragging the feet. Originally called tobogganing, skeleton was invented in St. Moritz, Switzerland, in the late 19th cent. It was an event in the Winter Olympic Games in 1928 and 1948, when the games were held in St. Moritz, and again in 2002.

Winter sport similar to lugeing in which a small sled is ridden downhill in a headfirst, prone position. The sport of skeleton sledding developed in the 1880s on the famed Cresta Run at Saint Moritz, Switz. The “bony” look of the early sleds gave the sport its name. Riders attain speeds of more than 80 miles (129 km) per hour. It was first contested at the Olympic games in 1928.

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Bony framework of the body. It includes the skull, vertebral column, collarbone, shoulder blades, rib cage, pelvic girdle and the bones of the hands, arms, feet, and legs. The skeleton supports the body and protects its internal organs. It is held together by ligaments and moved at the joints by the muscles, which are attached to it. The skeletal system includes both bones and cartilage.

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