Eardrum

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The tympanic membrane, or eardrum, is a thin membrane that separates the external ear from the middle ear. Its function is to transmit sound from the air to the ossicles inside the middle ear. The malleus bone bridges the gap between the eardrum and the other ossicles. Rupture or perforation of the eardrum can lead to conductive hearing loss.

Contents

Development

The eardrum forms from the joining of the expanding first pharyngeal pouch and groove. Around day 30 of gestation, the endoderm-lined first pharyngeal pouch expands to form the tympanic cavity, which subsequently envelops the inner ear ossicles. Simultaneously, the first pharyngeal groove, which is lined with ectoderm, expands to form the developing external auditory meatus. Separated by a thin layer of splanchnic mesoderm, the tympanic cavity and external auditory meatus join to form the tympanic membrane. As a result, the tympanic membrane is one of very few adult structures that is derived from all three germ layers.

Anatomy

The tympanic membrane is shaped like a flat cone pointing inwards towards the inner ear and is made up of three layers: 1) Cutaneum ~ skin 2) Radiatum, circulare ~ collagen fibres 3) Mucosum, epithelium.

Clinical aspects

When examining the tympanic membrane with an otoscope, a bright cone of light is seen in the anterior-inferior part of the membrane. This light is known as the "cone of light" or "light reflex". The tympanic membrane is separated into four quadrants, with the center of the four quadrants being the umbo. Nerves, specifically the chorda tympani nerve, and arteries pass through the layers of the superior portion of the membrane. Thus, when the tympanic membrane needs to be incised for medical procedures, ENT surgeons will always cut through the inferior and posterior part of the membrane to avoid the vasculature, nerves, and bones associated with the membrane.

The cartilagenous part forms one third of the canal. It has two deficiencies: the "Fissure of Santorini" in this part of cartilage and through them the parotid or superficial mastoid infections can appear in the meatus or vice versa.[1] It is one of the characteristic responses of an upper motor neuron lesion.

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