Tympanic membrane

Recommended video: Introduction to the ear [21:33]

Overview of the structures of the internal, middle, and external ear.

The tympanic membrane (eardrum) is an oval, semi-transparent structure situated between the external auditory meatus  and the tympanic cavity of the middle ear. 

The tympanic membrane is divided into two main parts: 

• Pars tensa - forms most of the tympanic membrane. It is thickened at its outer margin to form a fibrocartilaginous ring called the tympanic ring. 
• Pars flaccida (Shrapnell's membrane) - is a small triangular portion of the membrane situated superior to the lateral process of the malleus. More specifically, it is located between the anterior and posterior malleolar folds. 

The tympanic membrane also has two surfaces: the lateral (external) and the medial (internal) surface. The lateral surface of the tympanic membrane is concave towards the external auditory meatus and is directed anteriorly and inferolaterally. The medial surface of the tympanic membrane is convex towards the tympanic cavity. The point of maximum convexity of the membrane is known as the umbo which is created by the attachment of the malleus to the center of the membrane. When the tympanic membrane is illuminated under the light (otoscope), the concavity of the membrane produces a cone of light radiating from the umbo over the inferior quadrant of the membrane. 

The tympanic membrane receives a dual blood supply; the outer part receives blood via the deep auricular artery, while the medial surface receives blood via the anterior and posterior tympanic arteries. 

The membrane is innervated by three cranial nerves. The anterior portion of the lateral surface is innervated by the auriculotemporal nerve of the mandibular nerve (CN V3), while the posterior part is innervated by the vagus nerve (CN X). The medial surface is innervated by the glossopharyngeal nerve (CN IX).

The tympanic membrane serves as the physical barrier between the middle and outer ear and thus it has a role in the protection of the middle and inner ear structures. In addition, it also has an important role in sound transmission from the outer ear to the auditory ossicles.