The ear is composed of three parts, the outer ear, the middle ear, and the inner ear. The outer ear consists of the pinna, the part you see on the side of the head, and the outer ear canal. Sound enters through the outer ear canal and strikes the ear drum, which is the outermost boundary of the middle ear. The sound which strikes the ear drum is transmitted through three bones of the middle ear to the inner ear. The three bones are the malleus (hammer), incus (anvil), and stapes (stirrup). The footplate of the stapes fits into the oval window, which is the outermost boundary of the inner ear. The movement of the footplate causes fluid in the inner ear to move. The bones provide a mechanical advantage which moves the fluid.
Without that mechanical advantage, sound bounces off the fluid in the inner ear and the result is a serious loss of hearing. The inner ear is a fluid filled sac encased in bone, which gives it maximum protection from injury. The organ for hearing is the cochlea (pronounced “coke-lee-uh”). It contains approximately 20,000 cells which can sense movement of the fluid. When these cells sense movement of the fluid, impulses are generated in the nervous system. These impulses pass through the auditory nervous system and eventually reach the brain. The brain is the center for discriminating sounds from one another and eventually associating words with ideas. See an interactive picture of the parts that make up the ear.