The external acoustic meatus, commonly known as the ear canal, is a passageway that serves as the entry point for sound into the human auditory system. This tubular space begins at the external ear and ends at the tympanic membrane, or eardrum. It is a fundamental component of the outer ear, linking the environment to the middle and inner ear structures. The meatus functions both as a sound conduit and a protective barrier.
Location and Physical Structure
The external acoustic meatus is an S-shaped canal that extends from the concha, the hollow depression of the outer ear, to the tympanic membrane. In adults, this canal measures approximately 2.5 centimeters in length and has a diameter of about 7 to 9 millimeters. Its curved shape serves a protective role, helping to prevent foreign objects from traveling directly to the eardrum.
The canal’s structure is divided into two parts: an outer third and an inner two-thirds. The outer third is composed of elastic cartilage, which is continuous with the cartilage of the auricle. The inner two-thirds of the meatus is bony, passing through the temporal bone of the skull.
The entire passage is lined with skin, which is thinner in the bony portion. This skin adheres closely to the underlying structure and extends to cover the outer surface of the tympanic membrane. The outer section is more flexible, while the inner section provides rigid protection.
How Sound Travels Through the Meatus
The function of the external acoustic meatus is to serve as an acoustic conduit for sound waves. Sound waves collected by the auricle are funneled directly into this canal. The meatus then directs these vibrational energy waves toward the tympanic membrane, causing it to vibrate.
The unique shape and dimensions of the meatus cause it to behave like a tube open at one end and closed at the other. This physical configuration creates an acoustic resonance that selectively amplifies sound frequencies in a specific range. The canal boosts the sound pressure for frequencies around 3 kilohertz (kHz) by 30 to 100 times.
This amplification is significant because most human speech sounds are distributed within this 3 kHz bandwidth. By amplifying these specific frequencies, the meatus helps to increase the sensitivity of the human ear to the sounds relevant for vocal communication. The efficient transfer of sound energy to the eardrum is the first step in converting mechanical vibrations into nerve impulses that the brain interprets as sound.
The Protective Role of Earwax
Beyond its role in sound transmission, the external acoustic meatus performs a protective function through the production of cerumen, commonly known as earwax. Specialized ceruminous glands, which are modified sweat glands, and sebaceous glands are located within the skin lining the cartilaginous portion of the meatus. These glands work together to secrete cerumen, a yellowish substance composed of fatty acids, dead skin cells, and various secretions.
Cerumen provides a hydrophobic, or water-repellent, physical barrier that helps waterproof the ear canal and prevent water intrusion. Its sticky nature is effective for trapping dust, dirt, and foreign debris, preventing these particles from reaching the sensitive eardrum. Furthermore, cerumen contains antimicrobial components, such as lysozyme, which provide an antibacterial defense against microbial pathogens.
Small hairs are also present in the cartilaginous part of the canal, which assist in filtering the air and trapping particles. The ear canal has a sophisticated self-cleaning mechanism where skin cells migrate outward from the eardrum toward the meatus opening. This “conveyor belt” action naturally extrudes the cerumen, carrying the trapped foreign material out of the ear and eliminating the need for routine cleaning.