Anatomy and Physiology
The external ear is composed of the auricle and externalauditory canal. Both contain elastic cartilage derived from mesoderm and a small amount of subcutaneous tissue, covered by skin with its adnexal appendages (1,2). There is fat but no cartilage in the lobule. The auricle is derived from six hillocks, three each from branchial arches I and II (Fig. 135.1). During normal gestation, the cartilaginous hillocks merge to form the auricle, and with selectivegrowth of the mandible, the auricle rises from its original position near the lateral commissure of the mouth to the temporal area
The external auditory canal is derived from the first ectodermal branchial groove between the mandibular (I) and hyoid (II) arches (2,3). The epithelium lining this groove contacts the endoderm of the first pharyngeal pouch, thus forming the tympanic membrane, the mostmedial extent of external auditory canal. Connective tissue of mesodermal origin is found between ectoderm and endoderm and becomes the fibrous layer of the tympanic membrane (2). Because of its origin, the external auditory canal, including the lateral surface of the tympanic membrane, is derived from ectoderm and is lined by squamous epithelium.
The process of canalization is complete by aboutweek 12 of gestation, at which time the canal fills with epithelial tissue. The canal ordinarily recanalizes by about week 28 of fetal life (3).
The external auditory canal may be thought of in two sections. The outer 40% is cartilaginous and contains a thin layer of subcutaneous tissue between the skin and cartilage. The inner 60% is osseous, is formed primarily by the tympanic ring, and containsvery scant soft tissue between the skin, periosteum, and bone. The average length of the adult external auditory canal is 2.5 cm. Because of the oblique position of the tympanic membrane, the posterosuperior part of the canal is about 6 mm shorter than the anteroinferior portion (1). The junction of the cartilaginous and bony portions of the canal is a narrowed section termed the isthmus.
Thetragus and antitragus form a partial barrier to the entrance of macroscopic foreign bodies. Laterally to medially, the canal curves slightly superiorly and posteriorly in a gentle S shape. The canal can be thought of as pointing toward the nose; thus, the auricle needs to be pulled gently upward, outward, and backward to straighten the canal for examination. Three macroscopic defense mechanismsprotect the external auditory canal and lateral surface of the tympanic membrane: the tragus and antitragus, the skin with its cerumen coat, and the isthmus of the canal.
The skin of the cartilaginous canal contains many hair cells and sebaceous and apocrine glands such as cerumen glands (Fig. 135.2). Together, these three adnexal structures provide a protective function and are termed theapopilosebaceous unit. Glandular secretions combine with sloughed squamous epithelium to form an acidic coat of cerumen, one of the primary barriers to infection of the canal. An invagination of the epidermis forms the outer wall of the hair follicle, and the hair shaft forms the inner wall. The follicular canal is the space between these two structures. The alveoli of the sebaceous and apocrine 88glands empty into short, straight excretory ducts, which drain into follicular canals. Obstruction of any part of the ductal system predisposes to infection.
Figure 135.1 The auricle is formed from six auricular hillocks, three each from branchial archesI and II. |
The canal is normally a self-protecting and self-cleansing structure. The cerumen coat gradually works its way past the isthmus...