- Bones and cartilage form the skeleton of the human body. It gives the frame and shape to the body and helps in movement. It protects the inner organs.
- The skeleton comprises the skull, the back bone, ribs and the breast bone, shoulder and hipbones, and the bones of hands and legs.
- Ribs are the bones of the chest and they join the chest bone and back bone to form the rib cage. The back bone is made up of many small bones.
- Pelvic bones form the portion of the body below stomach.
- The bones are moved by alternate contractions and relaxations of two sets of muscles.
- Some additional parts of the skeleton that is not as hard as the bones and which can be bent. These are called cartilage.
- The bone joints are of various kinds depending on the nature of joints and direction of movement they allow. Joints can be bent or rotated.
- The rounded end of one bone fits into the cavity (hollow space) of the other bone. Such a joint allows movements in all directions.
- There are some bones in our head that are joined together at some joints. The bones cannot move at these joints. Such joints are called fixed joints.
- When contracted, muscle becomes shorter, stiffer and thicker. It pulls the bone. Muscles work in pairs. When one of them contracts, the bone is pulled in that direction. The other muscle of the pair relaxes. To move the bone in the opposite direction, the relaxed muscle contracts to pull the bone towards its original position, while the first relaxes. A muscle can only pull. It cannot push. Thus, two muscles have to work together to move a bone.
- Strong muscles and light bones work together to help the birds fly. They fly by flapping their wings.
- Fish swim by forming loops alternately on two sides of the body. The head and tail of the fish are smaller than the middle portion of the body – the body tapers at both ends. This body shape is called streamlined.
- Snakes slither on the ground by looping sideways. A large number of bones and associated muscles push the body forward. Snakes have a long backbone. They have many thin muscles. They are connected to each other even though they are far from one another. They also interconnect the backbone, ribs and skin. The snake’s body curves into many loops. Each loop of the snake gives it a forward push by pressing against the ground. Since its long body makes many loops and each loop gives it this push, the snake moves forward very fast and not in a straight line.
- The body and legs of cockroaches have hard coverings forming an outer skeleton. The muscles of the breast connected with three pairs of legs and two pairs of wings help the cockroach to walk and fly.
- Earthworms move by alternate extension and contraction of the body using muscles. Tiny bristles on the underside of the body help in gripping the ground.
- Snails move with the help of a muscular foot.
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