The pinnae of the external ear are cartilaginous frames that aid in focusing and localizing sound. Each pinna is anchored to the cranium by skin, cartilage, auricular muscles, and extrinsic ligaments. The anatomy of the pinna is illustrated in Figure 47–1.
The external auditory canal (EAC) is typically 24 mm in length with a volume of 1–2 mL. The lateral third of the canal is made of fibrocartilage, whereas the medial two thirds are osseous. During early childhood, the canal is straight, but takes on an “S” shape by the age of 9. The EAC has an important relationship with the mastoid segment of the facial nerve, which lies posterior to the EAC as it descends toward the stylomastoid foramen. The temporomandibular joint is anterior to the EAC, and disease processes affecting this joint may lead to otalgia.
The EAC is lined by stratified squamous epithelium that is continuous with the skin of the pinna and the epithelial covering of the tympanic membrane. The subcutaneous layer of the cartilaginous portion of the canal contains hair follicles, sebaceous glands, and ceruminous glands, and is up to 1 mm thick. The skin of the osseous canal does not have subcutaneous elements and is only 0.2 mm thick (Figure 47–2). The epithelium of the EAC migrates laterally, allowing the canal to remain unobstructed by debris. The rate of epithelial migration is 0.07 mm/d and is thought to occur at the basal cell layer.
Coronal section of the ear canal. The skin of the cartilaginous and osseous canals are magnified. (Reproduced, with permission, from Lucente F, ed. The External Ear. Copyright Elsevier, 1995.)
The ceruminous glands are modified apocrine sweat glands surrounded by myoepithelial cells; they are organized into apopilosebaceous units (Figure 47–3). Cerumen prevents canal maceration, has antibacterial properties, and has a normally acidic pH, all of which contribute to an antibacterial effect of cerumen.
Skin of the cartilaginous portion of the external auditory canal depicting apopilosebaceous units. (Reproduced, with permission, from Main T, Lim D. The human external auditory canal: An ultrastructural study. Laryngoscope. 1976;86:1164. Copyright LWW.)
The pinna is innervated laterally, inferiorly, and posteriorly by the great auricular nerve (cervical plexus). Arnold's nerve (a branch of the vagus nerve) innervates the inferior bony canal, the posterosuperior cartilaginous canal, and corresponding segments of the tympanic membrane and the cymba concha. The posterosuperior bony EAC is innervated by branches of the facial nerve. ...