The outer ear consists of the pinna, which is the part you can see on the side of your head, and the external auditory canal, which is the passage along which sound travels.
The middle ear is a space or cavity about 1.3 cm across that is filled with air. The cavity is connected to your nose and throat by the eustachian tube. A chain of three tiny bones called the ossicles stretches right across the middle ear cavity to conduct sound from the eardrum through to the inner ear. These bones are called the malleus, the incus and the stapes. When sound enters your ears and makes the eardrum vibrate, the vibrations pass from the eardrum along the ossicles.
The inner ear has two parts: the cochlea & the semicircular canals.
1. The cochlea – the hearing part – is a coiled spiral tube about 3.5cm long. The spiral contains two fluid-filled chambers; an outer and an inner one. The inner chamber contains the organ of corti. This has about 17,000 small hair cells, and each one has tiny hair-like structures called stereocilia, which project into thecochlear fluid. When sound waves enter the cochlea the stereocilia move, triggering an electrical pulse in the auditory nerve.
2. The semicircular canals are not used for hearing. They are part of your balance system. They are filled with fluid and have hair cells that project stereocilia, rather like those in the cochlea. They give your brain information about the direction your head is moving.