Skeleton, muscles and movement · Grade 5
Your skeleton is a frame of bones inside your body: 206 of them in an adult. It holds you up, protects the soft organs inside you and, with your muscles, lets you move. Muscles pull on bones across joints, and because muscles can only pull, they work in pairs.
Key facts
- The skeleton has four jobs: support, protection, movement and making blood cells (in the bone marrow).
- The skull protects the brain, the ribs protect the heart and lungs, and the backbone (made of vertebrae) protects the spinal cord.
- The femur (thigh bone) is the longest bone. Other bones: humerus (upper arm), radius and ulna (forearm), tibia and fibula (lower leg), pelvis (hips).
- Joints: hinge joints (elbow, knee) bend one way; ball-and-socket joints (shoulder, hip) move in all directions; fixed joints (skull) do not move.
- Tendons join muscles to bones. Ligaments join bones to bones at a joint. Cartilage cushions the ends of bones.
- Muscles can only pull, so they work in antagonistic pairs: to bend the arm the biceps contracts and the triceps relaxes; to straighten it the triceps contracts and the biceps relaxes.
- Animals with a backbone (vertebrates) have a skeleton inside (endoskeleton). Insects, spiders and crabs have a hard outer skeleton (exoskeleton). Worms and jellyfish have no hard skeleton.