Corpus: External acoustic meatus

The Meatus acusticus externus or external auditory canal is the connection between the eardrum and the surrounding area. Together with the auricle, it forms the outer ear. Colloquially, it is often referred to simply as the "auditory canal".

Macroscopy[Bearbeiten]

The meatus acusticus externus is the average 2 to 3.5 cm long passage from the outer ear opening (porus acusticus externus) to the eardrum, which marks the transition to the middle ear. It has a diameter of approx. 0.6 to 0.8 cm. The sagittal section shows an oval or elliptical cross-section.

The external auditory canal consists of 2 parts: The cartilaginous ear canal (Meatus acusticus externus cartilagineus), which comprises the outer third, and the bony ear canal (Meatus acusticus externus osseus), which comprises the inner two thirds.

Cartilaginous part[Bearbeiten]

The outer cartilaginous part is formed by the cartilage of the ear canal (cartilago meatus acustici), which merges continuously into the tragus and concha auricularis of the ear cartilage (cartilago auriculae). It is groove-shaped with an upward gap that is closed tubularly by tight connective tissue. At the end, the cartilage is connected to the bony porus acusticus externus by coarse connective tissue. There are also two fissures on the anteroinferior side of the cartilage, the incisurae cartilaginei meatus acustici (Santorini), which are also closed by connective tissue. The cartilaginous part is curved and forms an angle of approx. 145° to the transverse axis that is open towards the front. This serves as a protective device against injuries to the eardrum.

Bony part[Bearbeiten]

The inner bony portion is formed tubularly by the pars tympanica of the temporal bone. The notch, the incisura tympanica, remains on the upper edge and is covered by the pars squamosa of the temporal bone. Dehiscences may occasionally remain in the bone on the underside (Huschke foramen).

Innervation[Bearbeiten]

The sensory innervation of the anterior wall of the auditory canal is provided by the meatus acustici externi nerve of the auriculotemporal nerve. In addition, the vagus nerve sends a sensitive branch for the posterior wall, the ramus auricularis nervi vagi. The facial nerve and the glossopharyngeal nerve are also involved via nerve anastomoses.

Mechanical irritation of the external auditory canal can lead to reflex coughing and vomiting via the vagus nerve.

Blood vessels[Bearbeiten]

The external auditory canal is supplied with blood from the front via the anterior auricular rami from the superficial temporal artery and from the back via branches of the posterior auricular artery and the posterior tympanic artery. The profunda auricular artery from the maxillary artery (pars mandibularis) is an additional branch that supplies the bony part.

Lymphatic drainage[Bearbeiten]

The lymph vessels of the external acoustic meatus drain into the parotid, retroauricular and cervical superficial lymphatic nodes.

The lining consists of ear canal skin with multilayered keratinised squamous epithelium. In the area of the bony section, the narrow dermis is firmly attached to the periosteum. The firm adhesion of the skin to the perichondrium and periosteum is characterised by a high sensitivity to pain.

Special protective hairs, the so-called tragi, are found particularly at the outer entrance. The cartilaginous section contains sebaceous glands (Glandulae sebaceae) and tubular ceruminous glands (Glandulae ceruminosae). Together with shed epithelial cells, the glandular secretion forms a fat-rich secretion, the cerumen (earwax). This prevents the epithelium from drying out and the bitter substances it contains prevent the penetration of animals.

The bony section lacks glands and hair.

In addition to the protective function mentioned above, the external auditory canal serves to transmit sound. Resonance in the 3 kHz range improves hearing by 20 to 25 decibels.

The external auditory canal develops in the 4th embryonic week from the middle section of the 1st gill furrow. After one week, three mesenchymal ridges, the auricular tubercles (Tubercula auriculae), develop around this cleft-shaped depression in the neighbouring areas of the 1st and 2nd gill arches. These encircle the gill furrow in a dorsally closed arch. As the mesenchyme proliferates, the wall of the external auditory canal extends outwards to form the primary auditory canal, which later becomes the cartilaginous section.

The later bony section of the external auditory canal is formed by a complex process: there is initially direct contact between the epidermis of the primary auditory canal and the epithelium of the first gill pocket. The two epithelia are separated by ingrowing mesenchyme. Then the auditory canal epithelium proliferates to the external auditory canal plate (lamina epithelialis meatus acustici externi) and grows towards the lateral wall of the developing tympanic cavity. This gives rise to the long epithelial auditory cord (chorda epithelialis meatus acustici externi). In the 7th month, the central epithelial masses undergo apoptosis. This creates the lumen of the secondary auditory canal.

See main article: Embryonic ear development

Common diseases of the external auditory canal include:

• Otitis externa circumscripta
• Otitis externa diffusa

In the course of an otoscopy or the mere insertion of a cotton swab into the external auditory canal, vagal irritation is possible due to irritation of the ramus auricularis nervi vagi, which manifests itself, for example, as a coughing reflex.

A ceruminous plug (cerumen obturans) can obstruct the external auditory canal and impede sound conduction.