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من طرف KASIR السبت أكتوبر 18, 2008 7:25 pmSkeletal System
The skeletal system provides four basic functions:
The skeletal system provides four basic functions:
- Support for tissues and muscle
- Protection for vital organs
- Movement through bones and attached muscles
- Storage for minerals and immature blood cells
| Growth Ossification is the process by which bone is formed. Some bones (e.g. the flat bones of the skull) are formed in one stage from the connective tissue. This process is known as intramembranous or direct ossification. Other bones (e.g. short bones) are formed from the cartilaginous model of the future bone developed in the embryo, being dissolved and replaced by bone cells. This process is known as endochondral or indirect ossification - most bones are formed this way. Support Bones and cartilage that make up the skeleton are the only rigid materials in the body. The 206 bones of the skeleton provide a framework and points of attachment for many of the soft tissues of the body. The five main classifications of bones are : Long (e.g. femur), Short (e.g. tarsal bones of the foot), Flat (e.g. frontal bone of the skull), Irregular (e.g. vertebrae) and Sesamoid (e.g. knee cap) | Cross Section of a Long Bone |
Protection
These structures protect some of the vital tissues and functional organs of the body. Typical examples are:
- Skull - protects the brain
- Vertebrae - protects the spinal cord
- Thoracic cage - protects the heart and lungs
Movement
Bones act as levers during movement and provide solid structures to which muscles are attached. The joints allow movement between bones and these movements are directly related to the type of joint and range of motion. Joints fall into one of three categories: Fixed fibrous or Synarthroses (e.g. bones of the skull), Slightly moveable or Amphiarthroses (e.g. symphysis pubis) and Freely movable or Diarthroses.
Freely Movable joints comprises of four main groups: Ball and Socket (e.g. hip), Hinge (e.g. elbow), Pivot (e.g. radius and ulna) and Gliding (e.g. carpal joint of the wrist)
Component parts of a synovial joint
A fibrous capsule surrounds the joint and is strengthened by ligaments. The stability of these joints is dictated by the shape of articulating surfaces, their surrounding ligaments and muscles. For example, the knee is given great strength from 2 cruciate and 2 collateral ligaments. Whilst one of the hardest joints to dislocate is the hip. It is formed with the head of femur fitting neatly into the socket or acetabulum in the pelvis.
| Articular or hyaline cartilage covers and protects the ends of bones which meet to form a joint and therefore allows freedom of movement. It is a very hard, smooth material which does not repair itself when damaged. Tendons connect muscle tissue to bone and although more elastic than ligaments, have a far greater tensile strength than muscle. Synovial membrane lines the joint cavity and covers the tendons and ligaments which pass through it. The membrane produces synovial fluid which lubricates the joints | Shoulder Joint |
Ligaments are tough fibrous bands of tissue which connect bone to bone and help stabilise a joint, the strongest ligament in the body being situated at the front of the hip capsule, preventing excessive backward movement of the legs. Ligaments, although stronger than muscle tissue, have fewer nerve endings and less blood supply , and therefore take longer to repair when damaged. Whilst these strong fibrous bands offer great stability to a joint in preventing excessive movement, if they are stretched or torn through injury, they do not necessarily return to their former length and therefore may remain stretched, therefore offering reduced stability to that particular joint.
A bursa is a small sac formed in connective tissue lined by a synovial membrane and containing a small amount of synovial fluid. It is situated between moving parts, often between tendon and bone, to prevent rubbing.
Storage
In some bones, there is red marrow which produces red blood cells, some white cells and platelets. Minerals, especially calcium and phosphorous are also stored in bones and can be distributed to other parts of the body.
Effect of exercise on the skeletal system
The condition of bone may be improved by exercise as it responds to mechanical stresses. These mechanical stresses usually take the form of skeletal muscle pulling at their points of attachment being their origins and insertions. Where these mechanical stresses are applied, most it has been shown that more mineral salts are deposited and more collagenous fibres are produced. Therefore, both the density and size of bone in these areas may be increased and these changes in bone structure are stimulated by increased loads being placed on the skeleton. This has been borne out by greater bone mass being observed in weight lifters than in other lighter endurance athletes such as joggers. Other examples include racquet players who have been shown to have greater bone density in their playing arms. It has even been shown that if a leg is immobilised by being placed in plaster, due to a fracture, that even after a few weeks the bone becomes decalcified from lack of mechanical stress.
Whilst it may therefore be considered beneficial to utilise exercise to maintain healthy bones, great care must be taken with children whose bones and muscles are still developing. They should not be subjected to forms of sport involving high degrees of mechanical stress, partly because of the weaknesses that still exist within the bones, and also because of adverse effects on the development of these bones before maturity.
There are two main effects on bones as we grow older. Bones begin to lose calcium and this is one of the factors contributing to the condition called osteoporosis. Secondly, with age less protein is produced which alters the make-up of bone and sometimes creates brittle bones.
