The outer ear consists of the visible external ear or pinna, and the ear canal which ends at the eardrum.
The pinna's shape is not just useful for holding up our glasses and decorating with earrings. It actually collects sound and funnels it to the ear canal and down to the eardrum.
The ear canal is about 2.5 centimeters (1 inch) in length and 0.7 centimeters (0.3 inches) in diameter. You can see from the diagram that the structure beneath the skin of the ear canal is different near the ear canal opening from near the eardrum. Near the canal opening there is cartilage; as the canal approaches the eardrum there is bone. The ear canal is curved and lined with fine hairs and has small glands which produce cerumen (earwax), all of which protect the deeper structures beyond.
The eardrum, more formally known as the tympanic membrane, is the transition point between the outer ear and the middle ear. Below is an image of a healthy eardrum. You can see the translucent membrane through which one of the middle ear bones on the other side is clearly visible.
There are actually 3 small bones, called ossicles, connected together and suspended inside the air-filled middle ear space. The ossicles are commonly known as 'hammer', 'anvil' and 'stirrup'. The actual scientific names are 'malleus', 'incus', and 'stapes'. These are the smallest bones in the human body. They are connected together with the malleus attached to the tympanic membrane (the eardrum, shown below) on one end, the incus in the middle and the stapes attached to the inner ear on the other end.
There is also a tube, called the Eustachian tube, which connects the middle ear space and the throat. This tube opens periodically to equalize pressure on both sides of the eardrum. Sometimes you feel this equalization, like when your ears "pop" after an elevation change. Inflammation of this tube is usually the cause of a buildup of fluid in the middle ear space which may result in ear infections, especially in younger children.
The main function of the middle ear is to change the sound waves collected by the outer ear into mechanical vibrations and transfer them on to the fluid-filled inner ear.
The stapes is connected to the inner ear structure known as the cochlea, the primary organ of hearing. The cochlea is a small, snail shaped structure, which is filled with fluid. Inside the cochlea this fluid surrounds a variety of other smaller structures, most important of which are the hair cells. The cochlea is only about 30mm long but contains over 30,000 of these tiny hair cells, a few of which you can see in the image below/right. In fact, they are not hairs at all - they are highly specialized sensory cells with microscopic fibers - much smaller than hair.
Vibrations from the middle ear cause the fluid in the inner ear, and hence these cells, to move, converting sounds from a mechanical vibration into an electro-chemical signal which is then sent on to the hearing nerve and up to the brain.
Attached to the cochlea, and also a crucial part of the inner ear, is the vestibular labyrinth, one of the key components of our balance system.