The ear is concerned with two functions. The more evident is that of the sense of hearing; the other is the sense of equilibration and of motion. The organ is divided into three parts:
(1) the external ear, consisting of the auricle on the surface of the head, and the tube which leads inwards to the drum; (2) the middle ear, separated from the former by the tympanic membrane or drum, and from the internal ear by two other membranes, but communicating with the throat by the Eustachian tube; and (3) the internal ear, comprising the complicated labyrinth from which runs the vestibulocochlear nerve into the brain.
External ear The auricle or pinna consists of a framework of elastic cartilage covered by skin, the lobule at the lower end being a small mass of fat. From the bottom of the concha the external auditory (or acoustic) meatus runs inwards for 25 mm (1 inch), to end blindly at the drum. The outer half of the passage is surrounded by cartilage, lined by skin, on which are placed ?ne hairs pointing outwards, and glands secreting a small amount of wax. In the inner half, the skin is smooth and lies directly upon the temporal bone, in the substance of which the whole hearing apparatus is enclosed.
Middle ear The tympanic membrane, forming the drum, is stretched completely across the end of the passage. It is about 8 mm (one-third of an inch) across, very thin, and white or pale pink in colour, so that it is partly transparent and some of the contents of the middle ear shine through it. The cavity of the middle ear is about 8 mm (one-third of an inch) wide and 4 mm (one-sixth of an inch) in depth from the tympanic membrane to the inner wall of bone. Its important contents are three small bones – the malleus (hammer), incus (anvil) and stapes (stirrup) – collectively known as the auditory ossicles, with two minute muscles which regulate their movements, and the chorda tympani nerve which runs across the cavity. These three bones form a chain across the middle ear, connecting the drum with the internal ear. Their function is to convert the air-waves, which strike upon the drum, into mechanical movements which can affect the ?uid in the inner ear.
The middle ear has two connections which are of great importance as regards disease (see EAR, DISEASES OF). In front, it communicates by a passage 37 mm (1.5 inches) long – the Eustachian (or auditory) tube – with the upper part of the throat, behind the nose; behind and above, it opens into a cavity known as the mastoid antrum. The Eustachian tube admits air from the throat, and so keeps the pressure on both sides of the drum fairly equal.
Internal ear This consists of a complex system of hollows in the substance of the temporal bone enclosing a membranous duplicate. Between the membrane and the bone is a ?uid known as perilymph, while the membrane is distended by another collection of ?uid known as endolymph. This membranous labyrinth, as it is called, consists of two parts. The hinder part, comprising a sac (the utricle) and three short semicircular canals opening at each end into it, is the part concerned with the balancing sense; the forward part consists of another small bag (the saccule), and of a still more important part, the cochlear duct, and is the part concerned with hearing. In the cochlear duct is placed the spiral organ of Corti, on which sound-waves are ?nally received and by which the sounds are communicated to the cochlear nerve, a branch of the vestibulocochlear nerve, which ends in ?laments to this organ of Corti. The essential parts in the organ of Corti are a double row of rods and several rows of cells furnished with ?ne hairs of varying length which respond to di?ering sound frequencies.
The act of hearing When sound-waves in the air reach the ear, the drum is alternately pressed in and pulled out, in consequence of which a to-and-fro movement is communicated to the chain of ossicles. The foot of the stapes communicates these movements to the perilymph. Finally these motions reach the delicate ?laments placed in the organ of Corti, and so affect the auditory nerve, which conveys impressions to the centre in the brain.... ear
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