Skeletons and Muscles

–Karthik Gurumurthy

A sand castle quickly dissolves into a pile of sand if disturbed by a wave or the wind. What keeps an animal’s body from dissolving into a pile of cells? Its tissues are strong and hold the body together. Many animals also have a skeleton that gives their bodies structure and protects their organs.

Some animals have skeletons made from sturdy bone material inside their bodies. These animals are called vertebrates. (Each bone in your backbone is known as vertebra.)

Most animals do not have bones. They are called invertebrates(without backbones). Some do have a skeleton, but it is on the outside of their bodies, like armor. One kind of armor is called an exoskeleton (“exo” means outside). Exoskeletons give animals such as insects and millipedes their shape, structure and protection.

Other invertebrates have no skeleton. Some, such as clams and snails, grow a shell to hide inside, but others, such as worms and slugs, have no hard structure to protect their bodies.

Because humans are vertebrates, you have bones that extend through your back, legs, arms, fingers, and toes. Your bony rib cage surrounds your heart and lungs. Your brain is safely housed inside your hard, bony skull.

Most animals move by using muscles, tissues that can lengthen and shorten. In vertebrates, strong connective tissue called ligaments hold bones together, and tendons attach muscles to bones. When muscles shorten, they pull on the bones, making them move. When you bend your arm, the muscles on the front side of your elbow shorten, pulling your lower arm bone close to your upper arm. At the same time, the muscles on the back of your elbow stretch, allowing your arm to bend.