1. Lesson 3 What is the skeletal system?

2. You have seen a house being built? It starts as a frame, which supports the whole house.

3. Humans and other animals have a frame, too. This frame is called the skeleton.

4. Some animals like crabs and insects have a skeleton on the outside of their bodies. It is called an exoskeleton.

5. Humans and other vertebrates have a skeleton inside their bodies. An internal skeleton is called an endoskeleton.

6. The human skeleton is made mostly of bone. It also has some softer tissues called cartilage. Your outer ear is made of cartilege.

7. So is your nose. Squeeze them gently. Can you move them? Bend them?

8. Bone can’t move and bend like that. Cartilage also lines the inner surface of most joints. A joint is where two bones meet.

9. What do you think it would feel like if bone rubbed on bone every time you bent your elbow?

10. Bone rubbing on bone is very, very painful. That is why joints are lined with cartilage. Cartilage acts like a shock absorber for the bones. It cushions the joints.

12. The human body has 206 bones which do many jobs. Protection: Your brain, heart, and lungs are a few of your vital organs. Bones protect them from harm.

13. Movement: Some joints are movable. Other joints are not. The joints of your arms, legs, hands, and feet allow you to move around.

14. Most joints are held together by ligaments. Ligaments stretch easily. This allows bones to move easily. Bones and muscles work together to produce movement.

16. Blood cell production: Some bones have tube-like canals that are filled with a soft tissue called marrow. Marrow makes red and some white blood cells.