Why animals need a skeleton
A skeleton supports the body, protects soft organs (the skull protects the brain and the ribs protect the heart and lungs), makes blood cells in the bone marrow, and gives muscles something to pull on. Movement happens when muscles pull on bones across a joint.
| Joint | Movement | Example |
|---|---|---|
| Ball and socket | In all directions | Shoulder, hip |
| Hinge | Back and forth in one plane | Elbow, knee |
| Pivot | Rotation | Neck (atlas and axis) |
| Fixed | None | Skull bones |
Bones are joined to each other by ligaments, and muscles to bones by tendons. The ends of the bones in a movable joint are covered with smooth cartilage, and the joint is lubricated by synovial fluid.
Muscles work in pairs
A muscle can only pull as it contracts; it cannot push. So muscles work in antagonistic pairs. When one contracts, the other relaxes.
The arm as a lever
The forearm is a lever. The elbow is the pivot, the biceps pulls near the pivot and the load is in the hand. Because the muscle is close to the pivot, it must pull with a much larger force than the weight it lifts.
Hold a mass in the hand. The biceps is attached 4 cm from the elbow and the hand is 32 cm away.
Key terms
- Joint
- A place where two bones meet.
- Tendon
- A tough cord joining a muscle to a bone.
- Ligament
- A tough band joining one bone to another.
- Antagonistic muscles
- A pair of muscles with opposite actions.
Summary
- The skeleton supports, protects and gives attachment for muscles.
- Movement occurs at joints; tendons join muscle to bone and ligaments join bone to bone.
- Muscles only pull, so they work in antagonistic pairs.
- The arm is a lever and the biceps must pull with a force greater than the load.