External ear

The external ear refers to the outermost part of the ear, which is responsible for collecting sound and directing it into the auditory system. It is made up of two primary components: the auricle (or pinna) and the external auditory canal (also known as the ear canal). The auricle is the visible portion, composed mainly of cartilage and covered by skin, while the ear canal extends inward toward the tympanic membrane (eardrum). The external ear plays a crucial role in protecting the middle and inner ear by acting as the entryway for sound waves.

Location

The external ear is located on the lateral sides of the head, just above the level of the jaw and positioned slightly toward the front of the temporal bone. The auricle is visible on both sides of the head, while the external auditory canal begins at the entrance of the ear opening and extends inward, ending at the eardrum. The external ear connects the outside environment to the deeper structures of the ear.

Structure and Anatomy

The external ear consists of two main components: the auricle (or pinna) and the external auditory canal. Together, they form the outermost part of the ear that captures sound and channels it toward the deeper auditory structures. This section outlines the detailed anatomy of each component of the external ear.

Auricle (Pinna)

The auricle is the visible, external part of the ear made primarily of elastic cartilage, covered by skin. Its complex shape consists of several ridges and depressions that help capture and direct sound waves. The auricle is composed of various anatomical landmarks:

• Helix: The outer curved rim of the auricle, which starts from the root of the auricle and extends around the perimeter.
• Antihelix: Located inside the helix, the antihelix is a Y-shaped ridge that runs parallel to the helix. It separates the deeper depressions of the auricle.
• Concha: The central, bowl-shaped depression of the auricle that leads directly into the external auditory canal. The concha can be divided into two parts: the cymba conchae (upper) and cavum conchae (lower).
• Tragus: A small cartilaginous projection located anterior to the concha, partially covering the entrance to the external auditory canal.
• Antitragus: Positioned opposite the tragus, the antitragus is another cartilaginous projection located just above the lobule.
• Lobule: The soft, fleshy, and non-cartilaginous lower part of the auricle, commonly referred to as the earlobe.

The auricle is connected to the temporal bone via ligaments and is stabilized by small intrinsic and extrinsic muscles, which are vestigial in humans and provide minimal movement capability.

External Auditory Canal

The external auditory canal (also known as the ear canal or external acoustic meatus) is a tubular structure that extends from the concha of the auricle to the tympanic membrane (eardrum). The canal is approximately 2.5 cm long in adults and is divided into two sections:

• Cartilaginous Portion: The lateral third of the canal is composed of elastic cartilage, continuous with the auricle. This part is lined with skin containing hair follicles, sebaceous glands, and ceruminous glands that produce earwax (cerumen), which helps protect the ear.
• Bony Portion: The medial two-thirds of the canal is formed by the bony structure of the temporal bone. This portion is narrower and covered by a thin layer of skin, without the presence of glands or hair. It leads directly to the tympanic membrane.

The canal follows an S-shaped curve, first moving upward, then inward and downward, providing some protection for the tympanic membrane by preventing direct exposure to external objects.

Tympanic Membrane

The tympanic membrane, commonly known as the eardrum, forms the boundary between the external ear and the middle ear. Although it is technically part of the middle ear, it marks the end of the external auditory canal. It is a thin, semi-transparent membrane that vibrates in response to sound waves, transmitting these vibrations to the middle ear. The tympanic membrane is composed of three layers: the outer layer (continuous with the skin of the ear canal), the middle fibrous layer, and the inner mucosal layer.

Blood Supply

The blood supply to the external ear comes from several arteries:

• Posterior auricular artery (a branch of the external carotid artery), which supplies the auricle.
• Superficial temporal artery, which also contributes to the auricle’s blood supply.
• Deep auricular artery (a branch of the maxillary artery), which supplies the external auditory canal.

The venous drainage of the external ear follows the arterial supply, draining into the external jugular vein.

Nerve Supply

The external ear receives its nerve supply from several sources:

• Auriculotemporal nerve (a branch of the mandibular division of the trigeminal nerve), which supplies the anterior portion of the auricle and parts of the external auditory canal.
• Great auricular nerve (a branch of the cervical plexus), which supplies the posterior and inferior parts of the auricle.
• Auricular branch of the vagus nerve (Arnold’s nerve), which innervates parts of the external auditory canal and may cause a cough reflex when stimulated.
• Facial nerve also provides some innervation to the small intrinsic muscles of the auricle.

Lymphatic Drainage

The lymphatic drainage of the external ear is divided into different regions:

• Superficial parotid lymph nodes drain the anterior portion of the auricle and external auditory canal.
• Mastoid lymph nodes and deep cervical lymph nodes drain the posterior part of the auricle and the deeper sections of the ear canal.

Adjacent Structures

• Temporal Bone: The external ear is closely associated with the temporal bone, which houses the bony portion of the external auditory canal. The temporal bone also forms part of the skull surrounding the auricle.
• Mastoid Process: Located just behind the auricle, the mastoid process is a bony prominence of the temporal bone that serves as an attachment point for muscles of the neck and is involved in the drainage of the middle ear.

Function

The external ear plays a crucial role in the auditory system, primarily by collecting sound and protecting the internal ear structures. Its unique anatomy enables it to serve multiple functions that are essential for hearing and ear health. Below is a detailed explanation of the functions of the external ear.

Sound Collection and Amplification

The most important function of the external ear is to collect and amplify sound waves from the environment. The shape of the auricle (pinna) helps funnel sound waves into the external auditory canal. The ridges and folds of the auricle, particularly the helix and antihelix, help capture sound waves and direct them toward the ear canal. This focusing of sound waves allows for better hearing, especially of sounds coming from the front or side.

The external auditory canal further amplifies sound waves as they travel toward the tympanic membrane (eardrum). The canal’s curved, tubular shape acts as a resonator, amplifying frequencies between 2,000 and 5,000 Hz, which is critical for human speech perception. The canal length and structure ensure that sound is concentrated before reaching the eardrum.

Sound Localization

One of the key functions of the external ear is assisting with sound localization, which helps the brain determine the direction from which a sound is coming. The shape of the auricle plays an essential role in filtering and modifying sound waves based on their point of origin.

• Vertical Sound Localization: The folds and contours of the auricle, particularly the concha and the tragus, help differentiate whether sounds are coming from above or below. Sounds arriving from above may bounce differently off the auricle compared to sounds from below, creating subtle variations in frequency and timing that help the brain distinguish vertical sound sources.
• Horizontal Sound Localization: The external ear helps in determining whether sounds come from the left or right. The auricle and the positioning of the head create a difference in sound intensity and timing between the two ears, known as interaural time difference and interaural level difference. These differences assist the brain in detecting the horizontal direction of sound.

Protection of the Ear Canal and Tympanic Membrane

The external ear provides critical protection to the more delicate structures of the middle and inner ear. The external auditory canal acts as a barrier against foreign objects, debris, and pathogens. This protection is enhanced by the anatomical features and biological defenses of the ear:

• S-Shaped Canal: The curved structure of the ear canal serves as a natural guard, preventing larger objects and debris from directly reaching the eardrum. The canal’s shape helps minimize the impact of potential external damage.
• Cerumen Production: The skin lining the cartilaginous portion of the external auditory canal contains ceruminous and sebaceous glands that produce earwax (cerumen). Cerumen traps dust, debris, and microorganisms, preventing them from reaching the eardrum. It also has antimicrobial properties that help protect the ear from infections.
• Hair Follicles: The hair in the outer portion of the ear canal acts as a physical barrier, preventing foreign objects, such as insects, from entering the canal.
• Temperature Regulation: The external ear, particularly the auricle, plays a minor role in protecting the internal ear structures from extreme temperatures. The vascularized skin and cartilage of the auricle help moderate the temperature changes in the ear, preventing damage to the sensitive eardrum and middle ear structures.

Enhancement of Sound Quality

The external ear contributes to the enhancement of sound quality. Due to the unique shape of the auricle and the resonance properties of the external auditory canal, the external ear improves sound clarity and volume. The amplification of specific frequencies, especially in the range of human speech (around 2,000-5,000 Hz), is important for communication. This amplification makes sounds clearer and more understandable before they reach the tympanic membrane.

The canal’s resonant properties filter out lower and higher frequencies, thus improving the quality of the sound being transmitted into the ear. This allows the listener to focus more on important sound frequencies, such as human speech, while reducing background noise.

Self-Cleaning Mechanism

The external auditory canal has a natural self-cleaning mechanism. As the skin lining the canal grows outward, it slowly moves cerumen (earwax) and any trapped debris toward the external opening of the ear. This process helps clear the ear canal of accumulated wax and dirt without the need for manual intervention. The action of talking and chewing also aids in this process by promoting movement of the ear canal walls, assisting in the gradual removal of cerumen and debris.

Limited Role in Auricular Movements

Although minimal in humans, the auricle possesses intrinsic and extrinsic muscles that allow slight movement of the ear. These muscles are vestigial in humans, meaning they have lost much of their function over time. However, in some individuals, limited movement of the auricle is possible, allowing them to make small adjustments to the ear’s position. In many animals, these muscles are more developed and help in orienting the ears toward sound sources for better hearing. While this function is diminished in humans, it may have had a more critical role in ancestral species.

Clinical Significance

The external ear plays an important role in clinical contexts due to its exposure to environmental factors and its anatomical relationship with the middle and inner ear. Some of its clinical significance includes:

• Infections: The external ear is prone to infections such as otitis externa (also known as swimmer’s ear), which involves inflammation of the external auditory canal. This condition can be caused by bacteria or fungi, often due to excessive moisture or trauma to the canal.
• Trauma and Injury: Because of its location, the external ear is susceptible to trauma, including lacerations, burns, or contusions. Injuries like auricular hematoma (common in contact sports) can lead to permanent deformities such as cauliflower ear if not treated promptly.
• Congenital Deformities: Deformities of the external ear, such as microtia (underdeveloped auricle) or absence of the ear canal (atresia), can affect both appearance and hearing ability. Surgical interventions, such as ear reconstruction or canaloplasty, may be required.
• Earwax Impaction: Excessive production of earwax (cerumen) or improper cleaning techniques can lead to impaction, blocking the ear canal and causing hearing loss or discomfort.