External acoustic meatus

Recommended video: Introduction to the ear [21:33]

Overview of the structures of the internal, middle, and external ear.

The external acoustic meatus, also known as the external auditory canal or simply the ear canal, is a slightly S-shaped tube situated within the tympanic portion of the temporal bone. Its entrance is lies just in front of the mastoid process of the temporal bone.

In adults, this canal measures approximately 2-3 cm in length and extends medially and slightly upward to reach the tympanic membrane (eardrum), a thin, funnel-like layer of epithelial tissue that serves as the boundary between the outer and middle ear. Its primary function is to channel sound waves towards the tympanic membrane.

The lateral third of the external acoustic meatus consists of cartilage and is covered with skin that extends from the auricle, while the medial two thirds forms a bony tunnel covered with a thin layer of skin that is continuous with the external surface of the eardrum.

The narrow external opening of the external acoustic meatus prevents large objects from entering the canal and damaging the tympanic membrane. Within the subcutaneous tissue of the cartilaginous part, modified sweat glands known as ceruminous glands produce a wax-like secretion called cerumen. 

Cerumen, along with small hairs lining the ear canal, helps to capture foreign particles or tiny insects, preventing them from reaching the eardrum. Additionally, it moisturizes both the external acoustic meatus and the eardrum. Cerumen also contains lysozyme, an enzyme capable of breaking down the cell walls of bacteria.

The external acoustic meatus is supplied by the posterior auricular artery, the deep auricular branch of the maxillary artery, and the auricular branches of the superficial temporal artery. The corresponding veins lead to the external jugular and maxillary veins as well as the pterygoid venous plexus. Sensory innervation is by the auriculotemporal branch of the mandibular nerve and from the auricular branch of the vagus nerve.