2. Hearing is the sense by which sound is understood.

3. Like a wave in the ocean, a sound wave is a repeating disturbance or movement that transfers energy through matter or space.

4. Just as the crest of a water wave is followed by a trough, the compression part of a sound wave is followed by an area where the particles are far apart.

6. Sound waves can travel through solids, liquids, and gases. When the waves enter your ear, they stimulate nerve cells deep within your ear.

7. Messages are sent to your brain. When the sound reaches the hearing area of the cortex, it responds and you hear a sound.

8. 1. The outer ear 2. The middle ear 3. The inner ear

9. Your outer ear takes the sound waves and sends them down the ear canal to the middle ear.

10. The sound waves cause the eardrum to vibrate like the skin of a musical drum when you tap it. These vibrations then move through three tiny bones called the hammer, anvil, and stirrup.

11. The cochlea (KOH klee uh) is a fluid-filled structure shaped like a snail’s shell. When the stirrup vibrates, fluid in the cochlea begins to vibrate.

12. These vibrations bend hair cells in the brain by a nerve. Depending on how the nerve endings are stimulated, you hear a different type of sound.

13. Structures in your inner ear also control your body´s balance. Structures called cristae ampullaris (KRIHS tee. Am pyew LEER ihs), and the maculae (MA kyah, lee), sense different types of body movement. Both structures contain tiny hair cells.

14. As your body moves, gel-like fluid surrounding the hair cells moves and stimulates the nerve cells at the base of the hair cells. This produces nerve signals that are sent to the brain, which reads body movements. Then, the brain sends signals to skeletal muscles, resulting in body movements that maintain balance.