Clinical meaning
The ear is divided into three anatomic regions essential for systematic otoscopic and audiometric assessment. The external ear includes the auricle (pinna) and external auditory canal (EAC), a 2.5 cm S-shaped tube lined with skin, ceruminous glands, and hair follicles in its lateral cartilaginous portion, transitioning to a thin bony canal medially. The tympanic membrane (TM) separates the external from middle ear and consists of three layers: an outer squamous epithelial layer, a middle fibrous layer providing structural rigidity, and an inner mucosal layer continuous with the middle ear mucosa. The TM is divided into the pars tensa (larger, taut inferior portion) and pars flaccida (smaller, superior portion where cholesteatomas originate). Normal TM landmarks visible on otoscopy include the handle of the malleus (manubrium), the umbo (tip of malleus at the TM center), the lateral process of the malleus, and the cone of light (light reflex at 5 o'clock position in the right ear, 7 o'clock in the left). The middle ear is an air-filled cavity containing the ossicular chain (malleus, incus, stapes) that conducts sound vibrations from the TM to the oval window of the cochlea, amplifying sound by approximately 18-20 dB through the lever mechanism and area ratio effect. The eustachian tube connects the middle ear to the nasopharynx, providing ventilation, drainage, and protection. The inner ear houses the cochlea (hearing) and vestibular apparatus (balance). Sound transmission follows two pathways: air conduction (sound waves through EAC, TM, ossicles to cochlea) and bone conduction (vibrations through skull bones directly to cochlea, bypassing the outer and middle ear). This distinction is the basis of Weber and Rinne tuning fork tests: conductive hearing loss (outer/middle ear pathology) shows bone conduction greater than air conduction (negative Rinne) with Weber lateralizing to the affected ear, while sensorineural hearing loss (cochlear or CN VIII pathology) shows air conduction greater than bone conduction with Weber lateralizing to the unaffected ear.