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. The external auditory (or acoustic) meatus runs inwards for 25 mm (1 inch), to end blindly at the drum.

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 contains 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 fluid in the inner ear.

The middle ear has two connections which are of great importance as regards disease (see EAR, DISEASES OF). In front, the Eustachian tube joins the upper part of the throat and so keeps the pressure on both sides of the drum fairly equal.

This consists of a complex system of hollows in the substance of the temporal bone lined with a delicate membrane. Between the membrane and the bone is a fluid known as perilymph, while the membrane is distended by another collection of fluid known as endolymph. This membranous labyrinth consists of a hindmost part, concerned with the balancing sense, comprising a sac (the utricle) and three short semicircular canals opening at each end into it; and a forward part concerned with hearing, comprising another small bag (the saccule) and a still more important part, the cochlear duct. In the cochlear duct is placed the spiral organ of Corti, on which sound-waves are finally received and by which the sounds are communicated to the cochlear nerve, a branch of the vestibulocochlear nerve, which ends in filaments 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 fine hairs of varying length which respond to differing sound frequencies.

When sound-waves in the air reach the ear, the drum vibrates, 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 filaments placed in the organ of Corti, and so affect the auditory nerve, which conveys impressions to the centre in the brain.