16. The Human Ear: Structure and Function

The human ear is the sensory organ responsible for detecting sound waves and enabling the sense of hearing, and is conventionally described as being made up of three main parts: the outer ear, the middle ear, and the inner ear.

16.1 Outer Ear

The outer ear consists of the pinna (the visible, external, funnel-shaped flap of cartilage and skin on either side of the head), whose primary function is to collect sound waves from the surrounding environment and direct/funnel them inward, and the ear canal (auditory canal), a short tube-like passage through which the collected sound waves travel inward until they strike the eardrum (tympanic membrane) — a thin, tightly stretched, elastic membrane that forms the boundary between the outer ear and the middle ear, and which vibrates in response to the incoming sound waves striking it.

16.2 Middle Ear

The middle ear is a small, air-filled cavity located just beyond the eardrum, containing a chain of three tiny, delicately linked bones, collectively called the ossicles, individually named the malleus (hammer), incus (anvil), and stapes (stirrup) — these are, in fact, the three smallest bones in the entire human body. The vibrations of the eardrum are mechanically transmitted through this chain of three ossicles, one to the next, and in the process the amplitude (and effectively the force/pressure) of the vibration is significantly amplified before being passed on to the inner ear. The middle ear is also connected to the throat (the back of the nasal cavity) by a narrow passage called the Eustachian tube, whose function is to equalise air pressure between the middle ear cavity and the outside atmosphere (this is the reason the ears may feel 'blocked', and then suddenly 'pop' to relieve the sensation, during rapid altitude changes such as in an aircraft during take-off/landing, or in a fast-moving lift/elevator).

16.3 Inner Ear

The inner ear contains the cochlea, a small, fluid-filled, spirally coiled (snail-shell-shaped) structure that is the actual organ responsible for converting the mechanical vibrations received from the middle ear's ossicles into electrical nerve signals. Inside the cochlea, the incoming vibrations cause the fluid within it to move, which in turn stimulates thousands of tiny, highly specialised sensory hair cells lining its internal membrane; the stimulation of these hair cells generates electrical nerve impulses, which are then transmitted via the auditory (cochlear) nerve to the brain, where they are ultimately interpreted and perceived as the sensation of sound. The inner ear also contains the semicircular canals, three fluid-filled loops oriented in three different perpendicular planes, which are not involved in hearing at all but instead serve the separate function of detecting the head's rotational movement and orientation, thereby helping to maintain the body's sense of balance and equilibrium.