Anatomy of External Ear

The human ear is a important sense organ responsible for hearing and balance. It comprises three main parts: the outer ear, the middle ear, and the inner ear. Each part plays a vital role in collecting sound from the environment and transmitting it to the brain, where the speech and auditory centres process the information.

Functions of the Ear

• Hearing: The primary function of the ear is to collect sound waves, amplify them, and convert them into electrical signals that the brain can interpret. This process allows us to perceive and understand sounds from our surroundings.
• Balance (Equilibrium): The ear also plays a critical role in maintaining balance. The vestibular system in the inner ear detects changes in head position and movement, coordinating with the eyes and muscles to help maintain stability and avoid falls.

 Additional Roles

• Warning System: The ear helps detect potentially threatening sounds from the environment, acting as a warning system to alert us to danger.
• Communication: It is a major component of our communication system, enabling us to hear and interpret speech, music, and other sounds.

Anatomically, the ear is divided into three parts: the external ear, the middle ear, and the internal ear.

External Ear. The external ear consists of two main components:

• Auricle (Pinna): This is the visible outer portion of the ear that is attached to the lateral aspect of the head. It captures sound waves and directs them into the external auditory canal.
• External Auditory Canal: This canal extends from the auricle to the tympanic membrane (eardrum), which closes off the canal. It serves as a passageway for sound waves to reach the eardrum.

Middle Ear. The middle ear is an air-filled, mucous membrane-lined cavity located in the petrous part of the temporal bone. Middle ear lies between the tympanic membrane on the lateral side and the lateral wall of the internal ear on the medial side. The middle ear is connected to the nasopharynx by a narrow tube called the eustachian tube, which helps equalize air pressure in the middle ear. The middle ear contains a chain of three tiny bones called ossicles:

• Malleus (Hammer): Attached to the tympanic membrane, it transmits vibrations from the eardrum to the incus.
• Incus (Anvil): The middle ossicle that connects the malleus to the stapes.
• Stapes (Stirrup): The smallest bone in the body, which transmits sound vibrations from the incus to the inner ear through the oval window.

Internal Ear (Labyrinth). The internal ear, also known as the labyrinth, is responsible for converting mechanical signals from the external and middle ear into electrical signals that are transmitted to the brain. It consists of two main labyrinths: the bony labyrinth and the membranous labyrinth.

• Bony Labyrinth: This structure is divided into two main parts:
  • Vestibular Apparatus: Consisting of the vestibule and semi-circular canals, it contains sensory organs responsible for postural equilibrium and balance.
  • Cochlea: Shaped like a snail shell, it contains the sensory organ of hearing.
• Membranous Labyrinth: This structure is located within the bony labyrinth and includes the utricle, saccule, semi-circular ducts, and cochlear duct. It contains a clear fluid called endolymph. The space between the membranous and bony labyrinths is filled with perilymph.
  
  

 

The External Ear

The external ear comprises three main parts: (i) the auricle or pinna, (ii) the external acoustic canal, and (iii) the tympanic membrane.

(i) Auricle or Pinna

The auricle, or pinna, plays a important role in focusing and localizing sound waves from the environment. Apart from its lobule, the pinna consists of a single piece of yellow elastic cartilage covered with skin. The lobule contains adipose tissue, while the contour of the pinna is shaped by the configuration of its elastic cartilage. This cartilage also forms the outer one-third of the external acoustic canal.

In reconstructive surgery, cartilage from the tragus, perichondrium from the tragus or concha, and fat from the lobule are often used as grafts. Conchal cartilage is used to correct a depressed nasal bridge, and composite grafts of skin and cartilage are employed for repairing defects of the nasal ala.

There are two surfaces of the auricle: the medial (inner) and the lateral (outer). The skin on the lateral surface is tightly bound to the perichondrium, while it is loosely attached on the medial surface. The medial surface attaches to the skull and is less significant. The lateral aspect is concave, with various prominences and depressions.

Prominences and depressions of pinna:  

• The outside rim is called helix and it ends inferiorly at the lobule.
• A smaller curved rim, parallel and anterior to the helix, is the antihelix.
• The large hollow centre area is called concha.
• The external acoustic meatus starts from bottom of the concha.
• Just anterior to the opening of the external acoustic meatus and in front of the concha, is an elevation (the tragus).
• Opposite the tragus, is another elevation called antitragus.

Muscles of the Pinna

• Extrinsic Muscles (three in number): These include the anterior, superior, and posterior auricular muscles. They pass from the scalp or skull to the auricle and play a role in positioning of the auricle.
• Intrinsic Muscles (six in number): These muscles pass between the cartilaginous parts of the auricle and can change the shape of the auricle. However, they are small, inconsistent, and generally have no useful function.

The facial nerve innervates both groups of muscles.

Blood Supply of the Pinna

• Posterior Auricular Artery: A branch of the external carotid artery, it is the main artery supplying the medial surface (except the lobule), concha, middle and lower portions of the helix, and the lower part of the antihelix.
• Anterior Auricular Artery: A branch of the superficial temporal artery, it supplies the upper portions of the helix, antihelix, triangular fossa, tragus, and lobule.
• Small Auricular Branch from the Occipital Artery: This may assist the posterior auricular artery.

 

Blood Supply of Pinna

Venous drainage of the pinna is through vessels following the arteries.

Lymphatic Drainage of the Pinna

• Lymph from the medial surface drains to the lymph nodes at the mastoid tip.
• Lymph from the tragus and upper part of the lateral surface drains to the preauricular nodes.
• Lymph from the remaining auricle drains to the upper deep cervical nodes.

Nerve Supply of the Pinna

• Auriculotemporal Nerve (branch of the mandibular division of the trigeminal nerve, V3): Supplies the anterosuperior part of the lateral surface of the pinna, including the tragus and crus of the helix.
• Greater Auricular Nerve (C2, C3): A nerve of the cervical plexus, it supplies most of the outer medial surface of the auricle, the posterior part of the lateral surface, and the postauricular region.
• CN VII (Facial Nerve): Innervates the skin of the lateral concha, antihelix, lobule, and mastoid.
• CN X (Vagus Nerve): Its auricular branch (Arnold’s nerve) supplies the concha and postauricular skin.
• Lesser Occipital Nerve (C2): A nerve of the cervical plexus, it supplies the upper part of the medial surface of the auricle and the postauricular region.

(ii) The External Auditory Canal

The external auditory canal (EAC) extends from the bottom of the concha to the tympanic membrane, measuring approximately 24 mm along its posterior wall, with the anterior wall being 6 mm longer. The canal is wider laterally and narrows medially.

The EAC is S-shaped, with the cartilaginous part directed upwards, backwards, and medially, while the bony part is directed downwards, forwards, and medially. To properly examine the canal and tympanic membrane, the pinna must be pulled upwards, backwards, and laterally to align both parts. In neonates, the pinna is pulled downwards, backwards, and laterally due to the underdeveloped bony part.

Structurally, the EAC consists of two parts:

• Cartilaginous Outer (Lateral) One-Third Part: Measuring about 8 mm, it is formed by the same yellow elastic cartilage as the pinna.
• Bony Inner (Medial) Two-Third Part: Measuring about 16 mm, it is primarily formed by the tympanic part of the temporal bone, with the roof formed by the squamous part of the temporal bone.

Cartilaginous Part

The cartilaginous outer one-third is attached to the bony part of external auditory canal by fibrous bands. The skin over this section is thick and contains hairs and modified apocrine sweat glands (ceruminous and pilosebaceous glands) which produce earwax. The presence of hairs helps prevent foreign bodies from entering into the ear. Therefore furuncles (staphylococcal infection of hair follicles) are seen only in this part.  There are two horizontal deficiencies – the “fissures of Santorini” in this part increase flexibility but allow infections (e.g. malignant otitis externa) and neoplasms to spread to and from the parotid and temporomandibular regions.

Bony Part

The bony inner two-thirds is narrow, with its medial end marked by the tympanic sulcus, a groove that is absent superiorly. The skin here is thin and continuous with the tympanic membrane. The “isthmus,” about 5-6 mm lateral to the tympanic membrane, narrows the bony canal’s diameter, making it difficult to remove foreign bodies lodged medially. Beyond the isthmus, the anteroinferior part of the deep meatus dips forwards forming a wedge-shaped “anterior recess”. This recess acts as a gutter for ear discharge and debris and becomes a tough spot to access either in the clinic or at surgery. Wax is usually not seen in this part and if present over pars flaccida or attic region is rarely true wax.  It is almost always associated with an underlying cholesteatoma. Sagging in the posterosuperior part can indicate acute mastoiditis due to its relation with the mastoid antrum. The foramen of Huschke, found in the anteroinferior part, can persist into adulthood and, like the fissures of Santorini, allows infections and neoplasms to spread.

Relationships of EAC:-

• Superior: Middle cranial fossa
• Inferior: Parotid gland
• Posterior: Mastoid antrum and air cells and the facial nerve
• Anterior: Temporomandibular joint (TMJ)
• Medial: Tympanic membrane and middle ear
• Lateral: Outside world

Nerve Supply

• Auriculotemporal Nerve (CN V3): Supplies the anterior and superior walls.
• Auricular Branch of CN X (Vagus Nerve): Supplies the posterior and inferior walls.
• CN VII (Facial Nerve): Innervates the skin of the mastoid and posterior EAC.

Important Clinical Points

• Hitzelberger’s Sign: The hypoesthesia of posterior meatal wall occurs due to the pressure on facial nerve (sensory fibers are affected early) in patients with acoustic neuroma.
• Vasovagal Reflex: During EAC cleaning, stimulation of Arnold’s branch of the vagus nerve can cause coughing, bradycardia, syncope, and even cardiac arrest.
• Ramsay Hunt Syndrome: Vesicles of herpes zoster oticus on the mastoid and posterior meatal wall indicate facial nerve involvement.

——– End of the chapter ——–

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Learning resources.

• Scott-Brown, Textbook of Otorhinolaryngology Head and Neck Surgery.
• Glasscock-Shambaugh, Textbook of  Surgery of the Ear.
• Susan Standring, Gray’s Anatomy.
• Frank H. Netter, Atlas of Human Anatomy.
• B.D. Chaurasiya, Human Anatomy.
• P L Dhingra, Textbook of Diseases of Ear, Nose and Throat.
• Hazarika P, Textbook of Ear Nose Throat And Head Neck Surgery Clinical Practical.
• Mohan Bansal, Textbook of Diseases of Ear, Nose and Throat Head and Neck surgery.
   

  Author:

Dr. Rahul Bagla
MBBS (MAMC, Delhi) MS ENT (UCMS, Delhi)
Fellow Rhinoplasty & Facial Plastic Surgery.
Renowned Teaching Faculty
Mail: msrahulbagla@gmail.com
India

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