Where is Tympanic Membrane Located and What are its Dimentions?
Location: Tympanic membrane is a semitransparent partition between the external acoustic meatus(EAM) and tympanic cavity. It has following features:
- It is oval in shape.
- It is 9-10 mm tall, 8-9 mm wide and 0.1 mm thick.
- It is placed at an angle of 550 to the floor of EAM.
- It consists of following three layers (derived from the three germ layers):
- Outer cuticle layer – stratified squamous epithelium (develops from ectoderm)
- Middle fibrous layer (develops from mesoderm)
- Inner mucosal layer – columnar epithelium (develops from endoderm)
What are the Parts of Tympanic Membrane?
Tympanic membrane has thick margins which is attached to the sulcus in the tympanic plate of temporal bone. In the posterosuperior part the sulcus is replaced by a notch. From the ends of this notch two folds of membrane extend downwards to the lateral process of malleus. These folds are called anterior and posterior malleolar folds and they divide the tympanic membrane into two parts: The two parts are:
- Part tensa: It forms the lower larger lower part. Its periphery is thickened to form a fibrocartilaginous ring called the annulus tympanicus which fits in the tympanic sulcus.
- Part flaccida: It is the small triangular upper part situated between anterior and posterior malleolar folds. This part is lax due to the absence of fibrous layer.
Features of lateral surface of tympanic membrane
- It faces EAM and is concave.
- It is divided into four quadrants (anterosuperior, anteroinferior, posterosuperior and posteroinferior) by two imaginary lines. First imaginary line passes through the handle of malleus and the second passes through the tip of handle (at umbo) at right angles to the first line.
- A bright cone of light is seen in the anteroinferior quadrant.
Features of medial surface of tympanic membrane
- It is convex towards middle ear.
- The point of maximum convexity, where tip of handle of malleus is attached is called Umbo.
- Chorda tympani nerve is closely related to the upper part of this surface of tympanic membrane. It runs from posterior to anterior aspect medial to neck of malleus.
Nerve Supply of Tympanic Membrane
- Lateral surface is supplied by auriculotemporal nerve and auricular branch of vagus.
- Medial surface is supplied by glossopharyngeal nerve through the tympanic plexus.
Applied Aspect
On illumination, the normal tympanic membrane appears pearly grey in colour and reflects a ‘cone of light in its antero-inferior quadrant.
In myringotomy (incision in tympanic membrane) the surgical incision is placed in the posteroinferior quadrant to avoid injury to chorda tympani nerve.
Myringoplasty is repair of a perforation in the tympanic membrane by putting a graft. The graft is usually derived from the temporal fascia or cartilage of tragus of the patient.