The External Ear
The external ear has two parts: the "PINNA" which is the outside portion of the ear that is visible on the side of the head, and the "EXTERNAL AUDITORY MEATUS" (ear canal) that extends from the pinna to the "TYMPANIC MEMBRANE" (abbreviated TM and is commonly referred to as the eardrum). The pinna is mostly skin and cartilage with some muscular attachments to the back (allowing some talented folks to wiggle their ears). The pinna collects and directs sounds down the ear canal. The twists and folds of the pinna enhance high frequency (pitched) sounds and also help us to determine the direction of the sound source. Sounds coming from the front and sides are slightly enhanced as they are directed into the ear canal while sounds from behind are slightly reduced. This helps us to hear what we are looking at while reducing some of the distracting background noise coming from behind. Cupping the hand behind the pinna provides small additional amplification to sounds coming from the front because it effectively enlarges the sound collection surface area of the pinna. The external auditory meatus is a small, twisting, tunnel-like "tube" that connects the pinna to the tympanic membrane. It is about one inch in length (2.5 cm) and has a diameter about the size of a pencil eraser. The walls of the ear canal are sensitive to touch and there is even a cranial nerve branch that passes just below the back canal wall surface. If the ear canal is touched near this nerve, many people will react by coughing (Arnold's reflex). The outer half to two thirds of the canal is surrounded by cartilage and contains glands that produce "CERUMEN" (ear wax), while the inner one third to one half is surrounded by bone. The ear canal has a natural resonance that typically boosts sound pitches around 3000 "HERTZ" (abbreviated Hz. and is the same as cycles per second) by approximately 10-15 deciBels. Cerumen serves useful purposes in the ear canal that are discussed in a related article on cerumen management.
The Middle Ear
The middle ear consists of an air-filled space between the tympanic membrane and the inner ear that contains three tiny bones linked together that connect the tympanic membrane to an opening into the inner ear. It also contains tiny ligaments and muscles that support and adjust tension of the bony chain.
The TM is a concave shaped layer of membrane at the end of the ear canal. Sounds travel down the ear canal and strike the TM, causing it to vibrate. These vibrations are then transferred through the "OSSICLES" (the tiny bones previously mentioned) to the "COCHLEA" (inner ear). The first bone is the "MALLEUS" (hammer) and is attached to the inside surface of the TM. The second bone is the "INCUS" (anvil) and the innermost bone is the "STAPES" (stirrup). Sound sets this whole structure into vibration and the footplate of the stapes vibrates within the oval window opening of the cochlea, transferring sound energy to the fluids and tissues of the cochlea.
There is a small tube that connects the middle ear space to the back of the throat and is known as the "EUSTACHIAN TUBE." This tube is normally closed but opens momentarily with yawning, swallowing, etc. This periodic opening maintains equalization of the air pressure between the middle ear and the outside air pressure. This pressure must be equalized for most effective transfer of sound through the middle ear. If it becomes unequal as in relatively rapid altitude change, the sudden opening of the Eustachian tube produces a "pop" and improved hearing because pressure balance is restored.
The Inner Ear
The inner ear is comprised of two functionally separate sections: The "VESTIBULAR" or balance part and the cochlea, which is the hearing part. These two parts are interconnected and each serves its own vital function.
The purpose of the vestibular portion is to help us sense acceleration/deceleration of both rotational and linear motion and to sense head position in relation to gravity. Equally important, it is part of a reflex arc that makes it possible for us to maintain sharp visual focus with the many small and rapid motions of the head that occur as we engage in walking, riding, chewing, etc. Vestibular function is described in more detail in another article so will not be covered in further detail here.
The cochlea is a coiled canal in the dense bone tissue of the skull. The shape of this canal somewhat resembles a snail shell and houses three fluid-filled membranous canals extending its full length. The central canal houses the "ORGAN OF CORTI" which is comprised of specialized cells and their supporting tissues. Vibratory energy propagated through the fluid produces deformation of the organ of Corti, in turn resulting in shearing forces on tiny tufts of "hairs" or cilia extending from the upper surfaces of the "hair cells." This shearing action triggers an electro-chemical signal that travels upward through the auditory nervous pathway which passes through the internal auditory canal to the brainstem and then upward to the auditory processing centers in the temporal lobes of the brain. This is a simplified description of a fascinatingly complex activity that ultimately results in "hearing."