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Human skeletal system provides support, protects internal organs, and allows movement. The human skeletal system consists mainly of bones and cartilages (Figure 1A and 1B). Tendons, ligaments, and fibrous connective tissues provide additional support to the bones and ligaments.

Figure 49(A). Human skeletal system anatomy (Anterior view)
Figure 1A. Human skeletal system anatomy (Anterior view)

Figure 49(B). Human skeletal system anatomy (Posterior view)
Figure 1B. Human skeletal system anatomy (Posterior view)

Some animals, such as worms, do not have a skeleton. Other animals, such as insects, fish, amphibians, reptiles, and mammals, do have skeletons. Depending on location, skeleton system is divided into two types: internal or endoskeleton and external or exoskeleton (Figure 2).

Figure 2. Classification of skeletal system
Figure 2. Classification of skeletal system

Exoskeleton

An exoskeleton consists of a hard encasement on the surface of an organism (Figure 3). It provides defense against predators, supports the body, and allows movement. Animals such as insects possess external skeleton called chitin, a polysaccharide derivative of glucose.

Figure 51. Exoskeleton of Big horned beetle(Chalcosoma chiron )
Figure 3. Exoskeleton of Big horned beetle (Chalcosoma chiron )

Endoskeleton

An endoskeleton consists of hard, mineralized structure (bones and cartilages) inside of the organisms. Spicules of sponges (Figure 52) are examples of primitive endoskeletal structure. Cartilaginous fish (Figure 53) endoskeleton is made of cartilage. Adult human endoskeleton (Figure 49 A & B), consists of 206 bones cartilages, tendons, ligaments  and fibrous connective tissue.

Figure 4. Sponges and their spicules
Figure 4. Sponges and their spicules
Figure 5. Leopard torpedo ray (Torpedo panthera) has cartilaginous skeleton
Figure 5. Leopard torpedo ray (Torpedo panthera) has cartilaginous skeleton

Functions of Endoskeleton

Endoskeleton performs various functions mentioned below.

  1. Shape: It forms body shape and form of an individual.
  2. Support: One of the main functions of endoskeleton is to provide support to various parts of the body.
  3. Protection: It protects soft body parts.
  4. Muscle attachment and Movement: Large muscles attach to various parts of endoskeleton. These muscles contract and move bone as well as cartilage in turn bringing out movement of body parts and whole body.
  5. Reservoir: The maintain calcium and phosphorous levels of the blood.
  6. Breathing: Cartilages present in respiratory tract help in breathing.
  7. Hearing: Ear ossicles help in hearing by transmitting sound vibrations from tympanic membrane to internal ear.
  8. Blood cell formation: Bone marrow in bone produce RBC, WBC, and platelets (Figure 6).
Figure 6. Blood cell formation from differentiation of hematopoietic stem cells in red bone marrow.
Figure 6. Blood cell formation from differentiation of hematopoietic stem cells in red bone marrow