Basic anatomy and functions of the ear

Outer ear

The outer ear comprises the auricle, which helps us to perceive whether the source of a sound is behind or ahead. The presence of the auricle increases the pressure exerted on our drum by the sounds coming from the front by 5dB, which is the reason why we cup our ear when trying to hear better.

It contains outwardly directed hair designed to form a line of defense against small insects that try to enter the ear, the roots of which produce an oily liquid which gets mixed with secretions from adjacent sebaceous glands to form the basic ingredient of ear-wax (cerumen).

The medial and posterior thirds of the ear canal feature an osseous wall covered by a thin and rather brittle layer that lacks glands, while the distal extremity is covered by the tympanic membrane which forms the boundary between the outer and the middle ear.

Middle ear (Eardrum)

The tympanic membrane is a circular disc made of thin skin, 8-9mm in diameter, and shaped as an inward-looking cone. The middle ear itself lies inside the tympanic membrane and is an air-filled space containing three ossicles (malleus, incus, and stapes) that connect the tympanic membrane to the inner ear.

The malleus has a handle and a head and the handle is located between the layers of the tympanic membrane. The head of the malleus osculates an area of the middle ear called the tympanic cavity and hinges on the rather bulky body of the incus. From the incus stems a filament which turns to the lower middle ear and finally connects to the head of the stapes. The two limbs of the stapes are connected at the base, which is located in a small orifice, the oval window. Just below the oval window, there is another small orifice which leads to the inner ear, which is called the round window. A thin film seals this hole and when the stirrup base moves in-and-out, then the film of the round window moves out-and-in due to the presence of fluid in the inner ear which transmits the changes in pressure.

The facial nerve passes through the middle ear, on its way from the brain, through the cranium and to the facial muscles that control facial expression.

The middle ear also has two small muscles. One of these, the tensor tympani muscle, is located at the front and its function is not clear; it is possible that its aim is simply to make chewing and swallowing less noisy. The second, the stapedius muscle, is located at the back of the middle ear and responds to loud sounds by contracting and tightening the auditory ossicles’ chain, possibly reducing the transmission of prolonged and potentially damaging sounds to the inner ear.

Inner ear

The inner ear is perhaps the most complex part of the human body. It makes hearing possible by turning the sound into electrical signals, which then travel along the acoustic nerve to the brain. In addition, the inner ear plays a role in our sense of balance as, thanks to the labyrinth of the ear, we can perceive the acceleration of the head in any direction, either in a straight line or in a partial or full rotation or.

The part of the inner ear that actually “listens” is the cochlea. This tube is filled with a liquid, called perilymph, similar to that circulating throughout the body and the fluid surrounding the brain. Inside the perilymph lie the cochlear duct, the capillary cells and the nerve fibers that carry the signals to the brain.

Approximately half of the acoustic fibers pass over to the opposite side of the cerebral stem and then pass into the midbrain, to eventually convey the information to the cortex of the cerebral hemispheres where they become perceived.