Anatomy of External Ear

The human ear is an 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 three 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.
• Tympanic membrane (Ear Drum). 

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.

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.

(iii) Tympanic Membrane or the Drumhead.

The tympanic membrane forms a partition between the external acoustic canal and the middle ear. It is located at the medial end of the external auditory meatus and forms the majority of the lateral wall of the middle ear cavity. The tympanic membrane is set obliquely, which means the posterosuperior part of the tympanic membrane is more lateral than the anteroinferior part. The tympanic membrane measures 9–10 mm in height, 8–9 mm in width, and is about 0.1 mm thick. The tympanic membrane is slightly oval in shape, being broader at the top than the bottom, and forms an angle of approximately 55° with the floor of the external auditory canal.

The tympanic membrane is divided into two parts:

1. PARS TENSA. It is the lower part of the tympanic membrane and forms the majority of the tympanic membrane. The periphery of the tympanic membrane is thickened to form a fibrocartilaginous ring known as the annulus tympanicus, which fits into the bony tympanic sulcus. The bony tympanic sulcus makes an incomplete ring and does not extend into the notch of Rivinus at the roof of the canal. From the superior limits of the sulcus, the annulus becomes a fibrous band that runs centrally as the anterior and posterior malleolar folds toward the lateral process of the malleus. The remaining part of the bony ring is formed by the squamous part of the temporal bone. The central part is tented inwards at the level of the malleus, called the umbo, and a bright cone of light can be observed radiating from the malleus to the periphery in the anteroinferior quadrant of the tympanic membrane.
2. PARS FLACCIDA (Shrapnell’s Membrane). It is the upper part of the tympanic membrane and forms a small part of the tympanic membrane. It is situated above the lateral process of the malleus, between the notch of Rivinus and the anterior and posterior malleolar folds. The pars flaccida is less taut and often appears slightly pinkish.

Layers of the Tympanic Membrane. The tympanic membrane consists of three distinct layers:

• The outer epithelial layer. It is continuous with the skin lining the external auditory canal.
• The inner mucosal layer. It is continuous with the mucosa of the middle ear.
• The middle fibrous layer (lamina propria). It is present between the outer epithelial layer and inner mucosal layer. It encloses the handle of the malleus. It consists of radial, circular, and parabolic fibres. The arrangement of these fibres contributes to the complex pattern of tympanic membrane displacement during sound transmission. Note: In the pars flaccida, the middle fibrous layer (lamina propria) is less prominent, and the orientation of the collagen fibres is more random.

Blood Supply of the Tympanic Membrane. The arterial supply originates from branches supplying both the external auditory meatus and the middle ear.

• Lateral surface of tympanic membrane is supplied by the auricular branch of the maxillary artery.
• Medial surface of tympanic membrane is supplied by the anterior tympanic branches of the maxillary artery, the stylomastoid branch of the posterior auricular artery, and potentially from the middle meningeal artery.

Nerve Supply of the Tympanic Membrane

• Lateral surface of tympanic membrane. The anterior half of the lateral surface is innervated by the auriculotemporal nerve (V3). The posterior half of the lateral surface is supplied by the auricular branch of the vagus nerve (CN X), also known as Arnold’s nerve.
• Medial surface of tympanic membrane. It receives innervation from the tympanic branch of the glossopharyngeal nerve (CN IX), also known as Jacobson’s nerve.

Otoscopy. A normal tympanic membrane is shiny and pearly-grey in appearance. The transparency of the tympanic membrane can vary from person to person. Shadow of middle ear structures, such as the incudostapedial joint and round window can often be seen through the membrane.

Mobility (Seigalization). A healthy tympanic membrane is mobile, and its mobility can be tested using a pneumatic otoscope or Siegel’s speculum.

———— End of the chapter ————

Download full PDF Link:
Anatomy of External Ear Lecture Notes Dr Rahul Bagla ENT Textbook

Reference Textbooks.

• Scott-Brown, Textbook of Otorhinolaryngology-Head and Neck Surgery.
• Glasscock-Shambaugh, Textbook of Surgery of the Ear.
• 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
• Hans Behrbohm, Textbook of Ear, Nose, and Throat Diseases With Head and Neck Surgery.
• Salah Mansour, Middle Ear Diseases – Advances in Diagnosis and Management.
• Logan Turner, Textbook of Diseases of The Nose, Throat and Ear Head And Neck Surgery.
• Rob and smith, Textbook of Operative surgery.
• Anirban Biswas, Textbook of Clinical Audio-vestibulometry.
• Arnold, U. Ganzer, Textbook of  Otorhinolaryngology, 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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