The bony eac skin is different from the normal skin all over the body. It doesn't have hair secreting glands etc how is it histological different ? Does topical anaesthesia act on it ? (Lidocaine 4 % cotton pledgets )
The skin of the bony part of the auditory tube is thin, formed of four layered epidermis, as the stratum lucideum is absent, the dermis is highly vascular and easily bleed, the hypodermis ( fatty loose CT) is absent and the skin attached directly to the underlying periosteum.
Sensory innervation: Since it originates from branchial arch it is innervated by 5th, 7th, 9th and 10th cranial nerves. Auriculo temporal branch of the mandibular nerve innervates the anterior portion of the pinna, tragus, and the anterior wall of the external canal. The well of the concha and the posterior wall of the exterrnal canal receive innervation from the 7th, 9th, and 10th cranial nerves. This complex innervation of the external canal accounts for difficulty in doing local anesthesia of canal due to the different sources and nerves innervating it.
Histology:
The external canal is lined entirely by keratinising stratified squamous epithelium. This epithelium is in continuity with the lateral surface of the tympanic membrane. There is marked differences in the morphology of the skin as one progresses from medial to lateral in the external canal. The skin lining the bony canal is very thin, measuring about 30 - 50 microns in thickness. The rete ridges are absent in the skin lining the bony portion of the external canal. The skin here also lacks hair and other appendages. The skin here is loosely adherent to the underlying bone, facilitating easy elevation during surgery.
The skin over the cartilagenous canal is much thicker and more adherent than the skin of the bony canal. It has numerous hairs as well as sebaceous and ceruminous glands. There are no eccrine sweat glands in the external canal. The skin lining the external canal is the only keratinising epithelium that lacks eccrine glands. The hairs are most numerous at the lateral end of the canal, becoming less numerous medially and totally absent from the bony cartiagenous junction.
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