External Auditory Structure

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Pinna The external ear comprises of pinna and external auditory canal. The elastic fibrocartilage forms the body of the pinna and is covered by skin which is attached loosely on its medial surface. This cartilage is avascular and derives its nutrition from the perichondrium. A unique pattern on the lateral surface of the pinna makes it characteristic for each individual and comprises of helix, antihelix, triangular fossa, scaphoid fossa, concha, tragus, antitragus, intertragic notch and lobule. The narrow gap between the anterior crux of helix and tragus is deficient of cartilage and comprises of dense fibrous tissue, known as incisura terminalis. This serves as a site for endaural incision to access the temporal bone. (21,22) External Auditory…show more content…
It is the only membrane in the body that closes an orifice and remains intact. It is slightly oval shaped, emulates an irregular cone with its apex at the umbo and is obliquely placed. It forms the lateral wall of the tympanic cavity and subtends an angle of about 55° with the inferior wall of the external auditory canal, measuring 9-10 mm (horizontal dimension) x 8-9 mm (vertical dimension).(23) The thickened circumference forms the fibrocartilaginous tympanic annulus anchored in the tympanic sulcus that is deficient superiorly forming the notch of Rivinus. As the tympanomeatal flap is elevated, the annulus is taken along with the TM, and commences just above the notch of Rivinus superiorly. The division of TM into two unequal parts is carried out by the anterior and posterior malleolar folds into the triangular and smaller pars flaccida, and inferiorly pars tensa which is concave towards the…show more content…
This occurs by two different mechanisms. Firstly, the diminished TM surface area on which sound pressure is exerted results in dampened ossicular chain excursion. As perforation size enlarges, the loss of surface area is also greater. Hearing loss is greatest at lower frequencies and decreases as frequency increases. Additionally, the influence of site of perforation on degree of hearing loss is noted as more severe hearing losses are produced by posterior

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