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.
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