The ear is the anatomical organ that detects sound. It not only acts as a receiver for sound, but also plays a major role in the sense of balance and body position. The ear is part of the auditory system.
The word "ear" may be used correctly to describe the entire organ or just the visible portion. In most mammals, the visible ear is a flap of tissue that is also called the pinna and is the first of many steps in hearing. In people, the pinna is often called the auricle. Vertebrates have a pair of ears, placed symmetrically on opposite sides of the head. This arrangement aids in the ability to localize sound sources.
Audition is the scientific name for the sense of sound. Sound is a form of energy that moves through air, water, and other matter, in waves of pressure. Sound is the means of auditory communication, including frog calls, bird songs and spoken language. Although the ear is the vertebrate sense organ that recognizes sound, it is the brain and central nervous system that "hears". Sound waves are perceived by the brain through the firing of nerve cells in the auditory portion of the central nervous system. The ear changes sound pressure waves from the outside world into a signal of nerve impulses sent to the brain.
The outer part of the ear collects sound. That sound pressure is amplified through the middle portion of the ear and, in land animals, passed from the medium of air into a liquid medium. The change from air to liquid occurs because air surrounds the head and is contained in the ear canal and middle ear, but not in the inner ear. The inner ear is hollow, embedded in the temporal bone, the densest bone of the body. The hollow channels of the inner ear are filled with liquid, and contain a sensory epithelium that is studded with hair cells. The microscopic "hairs" of these cells are structural protein filaments that project out into the fluid. The hair cells are mechanoreceptors that release a chemical neurotransmitter when stimulated. Sound waves moving through fluid push the filaments; if the filaments bend over enough it causes the hair cells to fire. In this way sound waves are transformed into nerve impulses. In vision, the rods and cones of the retina play a similar role with light as the hair cells do with sound. The nerve impulses travel from the left and right ears through the eighth cranial nerve to both sides of the brain stem and up to the portion of the cerebral cortex dedicated to sound. This auditory part of the cerebral cortex is in the temporal lobe.
The part of the ear that is dedicated to sensing balance and position also sends impulses through the eighth cranial nerve, the VIIIth nerve's Vestibular Portion. Those impulses are sent to the vestibular portion of the central nervous system. The human ear can generally hear sounds with frequencies between 20 Hz and 20 kHz (the audio range). Although the sensation of hearing requires an intact and functioning auditory portion of the central nervous system as well as a working ear, human deafness (extreme insensitivity to sound) most commonly occurs because of abnormalities of the inner ear, rather than the nerves or tracts of the central auditory system.[1]
Outer ear (pinna, ear canal, surface of ear drum)
The outer ear is the most external portion of the ear. The outer ear includes the pinna (also called auricle), the ear canal, and the very most superficial layer of the ear drum (also called the tympanic membrane). In humans, and almost all vertebrates, the only visible portion of the ear is the outer ear. The word "ear" may properly refer to the pinna (the flesh covered cartilage appendage on either side of the head). This portion of the ear is very vital for hearing. The outer ear does help get sound (and imposes filtering), but the ear canal is very important. Unless the canal is open, hearing will be dampened. Ear wax (cerumen) is produced by glands in the skin of the outer portion of the ear canal. This outer ear canal skin is applied to cartilage; the thinner skin of the deep canal lies on the bone of the skull. Only the thicker cerumen-producing ear canal skin has hairs. The outer ear ends at the most superficial layer of the tympanic membrane. The tympanic membrane is commonly called the ear drum. The pinna helps direct sound through the ear canal to the tympanic membrane (eardrum).
The framework of the auricle consists of a single piece of yellow fibrocartilage with a complicated relief on the anterior, concave side and a fairly smooth configuration on the posterior, convex side. The Darwinian tubercle, which is present in some people, lies in the descending part of the helix and corresponds to the true ear tip of the long-eared mammals. The lobule merely contains subcutaneous tissue.[2] In some animals with mobile pinnae (like the horse), each pinna can be aimed independently to better receive the sound. For these animals, the pinnae help localize the direction of the sound source. Human beings localize sound within the central nervous system, by comparing arrival-time differences and loudness from each ear, in brain circuits that are connected to both ears. This process is commonly referred to as EPS, or Echo Positioning System.
The auricles also have an effect on facial appearance. In Western societies, protruding ears (present in about 5% of ethnic Europeans) have been considered unattractive, particularly if asymmetric. The first surgery to reduce the projection of prominent ears was published in the medical literature in 1881.
The ears have also been ornamented with jewelry for thousands of years, traditionally by piercing of the earlobe. In some cultures, ornaments are placed to stretch and enlarge the earlobes to make them very large. Tearing of the earlobe from the weight of heavy earrings, or from traumatic pull of an earring (for example by snagging on a sweater being removed), is fairly common.[3] The repair of such a tear is usually not difficult.
A cosmetic surgical procedure to reduce the size or change the shape of the ear is called an otoplasty. In the rare cases when no pinna is formed (atresia), or is extremely small (microtia) reconstruction of the auricle is possible. Most often, a cartilage graft from another part of the body (generally, rib cartilage) is used to form the matrix of the ear, and skin grafts or rotation flaps are used to provide the covering skin. Recently ears have been grown on a rat's back and attached to human heads after. However, when babies are born without an auricle on one or both sides, or when the auricle is very tiny, the ear canal is ordinarily either small or absent, and the middle ear often has deformities. The initial medical intervention is aimed at assessing the baby's hearing and the condition of the ear canal, as well as the middle and inner ear. Depending on the results of tests, reconstruction of the outer ear is done in stages, with planning for any possible repairs of the rest of the ear.[4][5][6]