skeleton

The human skeleton is made up of 206 bones and provides a strong but flexible supportive framework for the body.

The skull is a protective box for the brain, eyes, and hearing organs. It is also a framework for the teeth and flesh of the face. The cranium has eight bones: occipital, two temporal, two parietal, frontal, sphenoid, and ethmoid. The face has 14 bones, the main ones being two maxillae, two nasal, two zygoma, two lacrimal, and the mandible.

The human spine extends every night during sleep. During the day, the cartilage discs between the vertebra are squeezed when the body is in a vertical position, standing or sitting, but at night, with pressure released, the discs swell and the spine lengthens by about 8 mm/0.3 in.

Different types of skeleton. Vertebrate skeletons (top) are made of bone and cartilage and provide a scaffold for the flesh. Only in certain parts (for example the skull and the rib cage) are organs encased totally. A gorilla is shown here. Invertebrates may have an exoskeleton, made of chitin, as in insects and crustacea, such as the lobster (middle). The exoskeleton totally encases the animal and is periodically moulted to allow growth. Molluscs, worms, and other ‘soft’ invertebrates have a hydrostatic ‘skeleton’ that supports the body and facilitates locomotion. A cross-section of the fluid-filled cavity of an earthworm is shown below.

Framework of bones that supports and gives form to the body, protects its internal organs, and provides anchorage points for its muscles. It is composed of about 200 bones. Each bone is made of a mineral, calcium phosphate, and protein. Bones of the skeleton are joined to each other by ligaments. In the human body, walking, running, arm and leg movements, hand actions, and even just standing, are all achieved by the operation of muscles attached to bones of the skeleton. Movement of the body is brought about by the moveable joints of the body. The elbow joint is a good example. Muscles are attached to bones by tendons, and contractions of the muscle bring about movement.

The skeleton may be composed of bone and cartilage (vertebrates), chitin (arthropods), calcium carbonate (molluscs and other invertebrates), or silica (many protists). The human skeleton is composed of 206 bones, with the vertebral column (spine) forming the central supporting structure.

A skeleton may be internal, forming an endoskeleton, or external, forming an exoskeleton, as in the shells of insects or crabs. Another type of skeleton, found in invertebrates such as earthworms, is the hydrostatic skeleton. This gains partial rigidity from fluid enclosed within a body cavity. Because the fluid cannot be compressed, contraction of one part of the body results in extension of another part, giving peristaltic motion.

skeleton

Sport in which a single person drives a sled down an ice-track, lying head-first in a prone position. Skeleton was added to the Winter Olympic programme in 2002. Known in some parts of the world as tobogganing, riders can reach speeds of up to 130 kph/80 mph. Brakes are not allowed and steering is done by slight movements of the driver's body, or by dragging the feet.