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Human Body
11
the inner skeleton of the human body. In early life all bones are made of cartilage which is more flexible than bony tissue. Later on, it is replaced in all weight-bearing parts of the body. Cartilage is a tough but resilient, pliable form of compact connective tissue.
In the adults, cartilage is found in the nose, outer ear, larynx and airpassage in the adults. It is also found in the front parts of ribs and the moving surfaces of some joints. The transformation of cartilage into bone begins in later foetus-life, when calcium is desposited on a matrix made by the bone-forming cells. Until adolescence, a plate of cartilage cells remains near the ends of bones, enabling them to lengthen.
Bone, commonly thought of as a solid and inert substance, is really a living tissue. The hardness and rigidity of bone results from the deposits of inorganic calcium salts. There is a compact outer layer and a porous inner part in most of the bones. Some bones are hollow with a central cavity containing marrow. Periosteum is a fibrous covering containing cells which can form new bones to mend fractures.
Bones are joined together in a variety of diffetent ways. Some permit no movement, while some like the vertebral joints (with intervening discs) allow limited bending and rotation. Joints with a lubricated membrane and cartilagenous discs on moving surfaces permit free movement. (iv) Muscles
Muscles make up the bulk of soft tissues in the human body. Close to half of the body-mass is muscle. Three types of muscles are :
(i) Skeletal or strjated muscle (ii) Smooth muscle and (iii) Cardiac muscle.
Muscle cells have perfected contractility to an unparalleled degree. The contraction of muscle cells move body parts and the forces they can exert are phenomenal.
Skeletal muscles of the head, trunk and limbs are known as voluntary muscles. They are generally anchored at both
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