Skeleton and muscles
Biology · Key Stage 2 · The School
Three jobs at once
A skeleton does three things together: it SUPPORTS the body so you are not a heap on the floor, it PROTECTS the soft parts, and it lets you MOVE. The skull is a helmet for the brain. The ribs are a cage round the heart and lungs. The spine is a flexible column that shields the nerves running down your back.
Muscles pull, they never push
This is the fact most people get wrong. A muscle can only shorten and pull; it cannot push a bone back. So muscles work in ANTAGONISTIC PAIRS. Put a hand on your upper arm and bend your elbow: the biceps shortens and pulls the forearm up. Straighten it and the biceps relaxes while the triceps underneath shortens and pulls it back down.
Not everything has bones inside
Animals with a backbone are VERTEBRATES — mammals, birds, fish, reptiles, amphibians. Insects, snails and worms are INVERTEBRATES. An insect wears its skeleton on the outside, which is why a beetle is crunchy, and why it has to shed the whole thing to grow.
Put your right hand on your left biceps and bend your left arm. The biceps hardens and shortens — it is contracting and pulling. Now feel the back of your upper arm, the triceps: it is soft. Straighten the arm and swap: triceps hard, biceps soft. Neither muscle ever pushes. That is why they come in ANTAGONISTIC pairs — one to bend the joint, one to straighten it again.