Tympanic membrane is also known as the ear drum. Anatomically it could be considered to be a part of the external ear since it is attached to the medial terminal end of the bony meatus. Functionally speaking it is part of the tympanic cavity.
It is more or less oval in shape (egg shaped). It is 9mm in diameter. Its broad portion lies superiorly. It is pearly white in color, thin and semitransparent. When viewed under illumination a trianglular cone of light (reflected light) is seen extending from the centre forwards and downwards. This reflection, or cone of light is due to the sectional shape of the membrane. The ear drum is set with an obliquity of about 55 degrees to the floor of the external meatus. The centre of the ear drum appears retracted, and is known as the umbo. This umbo lies at the apex of the cone of light. Visible as an ivory colored extension upwards from the umbo is the handle of the malleus. If the posterior portion of the membrane is transparent, then the image of the long process of the incus, and occasionally the stapedial tendon may also be seen.
Ear Drum showing its various landmarks
The ear drum is composed of 3 layers. The outer layer is formed by stratified squamous epithelium, and is continuous with that of the external auditory canal. Any condition affecting the skin of the external canal will also affect the outer layer of the ear drum. Common conditions like dermatitis involving the skin of the external canal can also involve the outer layer of the ear drum. Embryologically outer layer of the ear drum developed from the ectoderm. Myringitis granulosa a common condition affecting the ear drum affects only the outer layer of the tympanic membrane. The middle and inner layers are not involved in this condition. It is commonly caused by infections arising from the external canal. Constant irritation of the ear drum due to presence of wax may also predispose to this condition. Another condition which involves the outer layer of the ear drum is Bullous myringitis. In this condition blebs may be seen in the outer layer of the ear drum. It is commonly caused by viral infections, or mycoplasma pneumonia. It may also be associated with middle ear effusion.
Otoendoscopic view or retracted ear drum showing middle ear contents
The middle fibrous layer from which the ear drum derives its strength and resilience is derived from the mesoderm. This portion is infact sandwiched between the outer squamous lining derived from the ectoderm and inner mucosal lining of the middle ear cavity derived from the endoderm. The ectodermal and mesodermal components of the ear drum arise from the first branchial cleft, while the endodermal component is derived from the pharyngotympanic recess. The middle fibrous layer has two components: 1. radial and 2 circular fibres. The handle of the malleus lie between the middle fibrous layer and the inner mucosal layer of the ear drum. From the handle of the malleus the radial fibres of the middle fibrous layer radiate towards the circumferance of the ear drum. The circular fibres are more prominent and thickened along the circumference of the ear drum. The condensation of the circular fibres are fixed to the tympanic sulcus at the medial end of the external auditory canal. This middle firbous layer is absent in the attic area of the ear drum. The fibrocartilagenous ring and the fibrous layer of the ear drum are deficient superiorly. This deficient area is known as the notch of Rivinus. The attic portion of the ear drum which lack the middle layer is known as the pars flaccida, while the rest of the drum which has all the three layers is known as pars tensa. The chorda tympani nerve which is a branch of the facial nerve run between the middle fibrous and inner mucosal layers of the ear drum.
Figure showing embryology of ear drum
The skin of the external canal and the outer lining of the tympanic membrane are unique in a sense that they lack frictional and abrasive contacts which is common with the skin lining elsewhere in the body. Desquamated keratin does not accumulate on the surface of the tympanic membrane, or in the deep external meatus, because the skin lining here is endowed with a peculiar feature known as Migration. The surface layers of the skin of the ear drum, and the surface keratin move towards the periphery of the membrane, and then slowly along the external meatus to the exterior. Derangements of this unique feature is associated with some of the diseases of the external ear.
The inner layer of the ear drum derived from the endoderm of the pharyngotympanic recess is continuous with that of the mucosal lining of the middle ear cavity.
The external surface of the ear drum receives its blood supply from the deep auricular branch of the maxillary artery. This small artery leaves the first part of the maxillary artery behind the neck of the mandible and gains access into the external canal by piercing the anterior wall behind the mandibular joint. It sends small branches into the membrane from the whole circumference of the pars tensa and one or more manubrial branches that descend on the handle of mandible from above. The internal surface of the ear drum is supplied from behind by the stylomastoid branch of the posterior auricular artery, and from the front by the tympanic branch of the maxillary artery. The superficial veins open into the external jugular vein; and those on the internal surface drain into the transverse sinus and veins of the dura mater, and partly into the venous plexus on the eustachean tube.
The innervation of the posterior half of the ear drum is by the auricular branch of the X nerve and the anterior half is by the auriculotemporal branch of the Vth nerve. The inner surface of the ear drum is supplied by the tympanic branch of the IXth nerve.
Appearance of the ear drum in various diseases:
The normal ear drum is pearly white in color. In pathological states this color of the drum may change.
Is seen in acute otitis media and in glomus jugulare.. In acute suppurative otitis media discharge may be seen extruding from a small perforation in the pars tensa portion of the ear drum. This is known as the light house sign.
Is seen in secretory otitis media, Highly placed jugular bulb
Seen in tympanosclerosis