Skeletal System
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10 The Skeletal System
What is Skeletal System?
10.1 Your skeletal system supports and protects the soft tissues and vital organs of your body. The skeleton is bony framework of any vertebrae (animal having a backbone). It gives the body shape and provides a system of levers operated by muscles that enable the body to move. If we did not have a skeleton, we would be floppy like a rag doll. Your heart and lungs are shielded by your ribs, your spinal cord is protected by your vertebrae, and your brain is protected by your skull. Without rigid bones and flexible joints, you would not be able to stand, sit, bend, walk, or run. By themselves, bones cannot move. Your skeletal system moves because muscles attached to your bones contract or relax in response to messages sent from the nervous system. Some of the skeletal bones are held together by strong fibers called ligaments.
The Skeleton
10.2 The human skeleton is divided into two main parts. The first part is made up of the bones of the head, neck and trunk. The spine supports the other parts of the body. The Chest ribs protect the heart and lungs. The ribs move up and down, controlling the movement of air in and out of the lungs. The second part is made up of the bones of the arms and legs and their supports. The shoulder consists of the shoulder blade and the collarbone. The leg is attached to the trunk by the pelvis, which is made up of two hipbones. The pelvis supports the lower part of the abdomen, surrounds the urinary bladder and the last portion of the large intestine. The two symmetrical hip bones of the pelvis form a basin-like structure. When we sit down, much of our weight rests on these bones.
Number of Bones
10.3 The skeleton contains about 275 different size and shape bones in a baby and 206 bones in an adult depending on how they are counted. The pelvis, for instance, can be counted as a single bone or as six bones fused together. Children have 33 vertebrae in their spine, but in adults the bottom five become fused to form the sacrum. There are 24 ribs in 12 pairs, and there are 56 phalanxes (finger and toe bones). The hard shell of the skull consists of 28 bones and provides protection for the brain and the delicate sense organs. The bones of the body vary greatly in size. The thigh bone, or femur, is the largest single bone in the body. The three smallest irregular bones – the stirrup, the anvil, and the hammer – are found in the ear and are less than one-half inch long.
Composition of Bones
10.4 They may seem hard and lifeless, but bones are made of living tissue. The outer membrane of the bone, called the periosteum, contains many nerves, as well as blood vessels that transport food and oxygen to the bone’s many cells. Inside the periosteum is the bony layer, which contains more blood vessels, bone cells (osteoblasts), and nerves. At the centre of some bones is the marrow, which makes blood cells and stores fat. Bones appear to be hard and dry, like stone. Yet they are lightweight. Mineral salts like the salts of calcium and phosphorus provide the hardness. Without these salts, bones would be as soft and pliable as rubber. Bone is the most active organ in the body. Bones are continuously reshaped, remodeled and overhauled.
