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Skeletal System (bones)

Updated on April 2, 2010


Skeleton is a flexible, bony framework of any vertebrate animal. It gives the body shape, protects the vital organs, provides a system of levers, operated by muscles that enables the body to move, houses the bone marrow (blood forming tissues), stores the elements sodium, cal­cium and potassium and releases to the blood, and holds the reserves pro­tein that the body uses during fasting.

Skeletal system is composed of two parts: axial skeleton (made up the bones of the head, neck and trunk) and the appendicular skeleton (made up of the bones of the arms and legs).

Animals with two legs are called bipeds while animals with four legs are called quadrupeds. Animals without backbones (invertebrates) like insects and lobsters have hard body coverings called exoskeleton while vertebrates have endoskeleton.

Exoskeleton versus Endoskeleton

The arthropods and the vertebrates are the most mobile of the multi­cellular animals. The difference between the skeletal systems of arthropods and vertebrates is that the arthropods have an exoskeleton a hard body covering with all muscles and organs located inside it. While vertebrates have an endoskeleton - a framework embedded within the organisms, with the muscle outside. Exoskeleton, which are composed of noncellular ma­terials secreted by the epidermis, function as a protective armor for the softer body parts and as a waxy barrier preventing excessive water loss by terrestial arthropods. The rib cage of the vertebrate endo-skeleton pro­tects the organs of the thorax, and the skull and the vertebral column protects the brain and spinal cord. Exoskeleton impose difficulties in over­all growth, and periodic molting of the exoskeleton and deposition of a new one are necessary to permit size increase. The exoskeleton impose limi­tation on the possible size increase of the animal.

Human Skeleton. There are 206 separate bones of the human skeleton. Bones are attached to the neighboring bones by joints. Bones fit together and held in place by strong bands of flexible tissue called ligaments.

Part of the Vertebrate Skeleton


Axial Skeleton (median)

Skull includes the cranium (brain box), sense capsule (nose, ear and eye) and visceral arches (jaws, hyoid and larynx).

Vertebral column includes the cervical (neck), thoracic (chest), lumbar (lower back), sacral (hip), and caudal (tail).

Thoracic basket includes the ribs and sternum (breast bone).

Appendicular Skeleton (lateral and paired)

Pectoral (shoulder girdle) scapula (dorsal)

clavicle  (anterior)

coracoid   (posterior)

Forelimb (arms) humerus (upper arm) radius and ulna (forearm) carpals (wrist) metacarpals (palm)

Pelvic (hip) girdle ilium pubis ischium Hindlimb (legs) femur (thigh)

tibia and fibula (shank) tarsals (ankle) metatarsals (sole)

pnalanges (fingers) phalanges (toes)




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