The rigid parts of an animal that serve to support or protect the soft parts of the body constitute the skeleton. There are two types of skeleton. (i) When the hard material is formed mainly on the outside of the body, it is called exoskeleton. The muscles are attached to the inner surface of exoskeleton (e.g., insects, crabs, crustaceans etc.). (ii) The endoskeleton is built within the body of the animals, surrounded by soft tissues, and have muscles attached to their outer surface. It is made of bone and cartilage (e.g., vertebrate skeleton).

In vertebrates (e.g., man), the skeleton forms the internal framework of the body. The skeleton is important and its basic functions are support, protection, movement and locomotion, muscle attachment, hemopoiesis (blood cell formation) and storage of calcium and phosphate in bones.

Figure 20.1 Principal subdivisions of the human skeleton