General structure of the organ of hearing
Anatomically, the organ of hearing and balance is conventionally divided into three major parts, each with its own specific components:
- External ear — includes the auricle and the external acoustic meatus.
- Middle ear — consists of the tympanic cavity and the auditory tube.
- Internal ear — represented by a complex system of osseous and membranous labyrinths.
In the topographic anatomy of the external ear, special attention is paid to the relief of the auricle and the spatial orientation of the acoustic meatus relative to key head structures.
Auricle (Auricula)
The auricle is a prominent duplication of skin. Within this skin fold lies an elastic cartilage, which determines the entire characteristic outer relief of the auricle.
Key elements of the auricle relief:
- Helix (helix) — a prominent outer rim bordering the auricle along its edge.
- Crus of helix (crus helicis) — a ridge projecting in the central part that divides the concha of the auricle into two specific anatomical fossae: the cymba conchae (cymba conchae) and the cavum conchae (cavitas conchae).
External acoustic meatus (Meatus acusticus externus)
The external acoustic meatus is a canal that conducts sound waves to the tympanic membrane. It has a complex shape, forming a curve directed anteriorly and inferiorly. The canal is structurally inhomogeneous and consists of two parts: the cartilaginous and osseous parts. At the transition from cartilage to bone, an anatomical narrowing of the canal is located.
Parameters and boundaries of the entrance: The entrance to the acoustic meatus (porus acusticus externus) has distinct cartilaginous boundaries. Anteriorly, it is covered by the tragus (tragus), and posteriorly, it is bounded by the antitragus (antitragus). The dimensions of the canal vary: the total length is 3 to 4 cm, the vertical dimension ranges from 0.9 to 1.1 cm, and the horizontal dimension is from 0.7 to 0.9 cm.
Topography of the acoustic meatus walls
Syntopy (neighborhood with other organs) of the external acoustic meatus is of paramount importance in operative surgery, as inflammatory processes can easily spread to adjacent structures.
The topography of the canal walls is as follows:
- Anterior wall — directly adjacent to the temporomandibular joint.
- Posterior wall — separates the canal lumen from the mastoid cells.
- Superior wall — serves as the boundary separating the meatus from the cranial cavity.
- Inferior wall — closely adheres to the parotid salivary gland.
Tympanic membrane (Membrana tympani)
The tympanic membrane is a crucial barrier that anatomically and functionally separates the external acoustic meatus from the middle ear structures.
It is located not perpendicular to the axis of the acoustic meatus, but obliquely. Due to this inclination, the tympanic membrane forms an obtuse angle with the superior wall of the external acoustic meatus, while forming an acute angle with the inferior wall of the meatus.
Briefly: answers to topical questions
Which blood vessels supply the auricle?
The blood supply to the auricle region is associated with branches of the external carotid artery.
- Posterior auricular artery (a. auricularis posterior) — branches off the external carotid artery, ascends along the posterior margin of the auricle toward the mastoid process; among its branches is the auricular branch (r. auricularis).
- Superficial temporal artery (a. temporalis superficialis) — a branch of the external carotid artery; emerges from the substance of the parotid salivary gland and ascends anterior to the tragus.
What are the innervation features of the skin of the auricle and external acoustic meatus?
The innervation of the skin of the auricle and external acoustic meatus includes branches of the trigeminal nerve and the cervical plexus.
- Auriculotemporal nerve (n. auriculotemporalis) — a branch of n. mandibularis; innervates the skin of the external acoustic meatus and the tympanic membrane.
- Great auricular nerve (n. auricularis magnus) — innervates the auricle and the skin behind and below it.
- Lesser occipital nerve (n. occipitalis minor) — innervates the skin of the region behind the auricle.
- Posterior auricular nerve (n. auricularis posterior) — a branch of the facial nerve; runs parallel and posterior to the attachment site of the auricle, heading toward the occipital belly of m. occipitofrontalis.
To which lymph nodes does lymph drainage from the external ear occur?
Lymph drainage from the structures of the external ear occurs into the mastoid lymph nodes (nodi lymphatici mastoideae).
- These nodes collect lymph from the posterior surface of the auricle, the external acoustic meatus, and the tympanic membrane.
- The mastoid lymph nodes are located superficially or deep to the sternocleidomastoid muscle (m. sternocleidomastoideus).
To memorize the topography of the acoustic meatus walls, use the association: superior — brain (cranial cavity), inferior — saliva (parotid gland), anterior — joint (temporomandibular), posterior — process (mastoid).
Class and exam questions
What structures make up the external ear?
The external ear includes the auricle and the external acoustic meatus. The tympanic membrane serves as the boundary between the external and middle ear.
Where is the narrowest part of the external acoustic meatus located?
The meatus narrows at the site of the anatomical transition from its cartilaginous part to the osseous part.
What do the superior and inferior walls of the external acoustic meatus border?
The superior wall separates the acoustic meatus from the cranial cavity, while the inferior wall borders the parotid salivary gland.
How is the tympanic membrane oriented relative to the walls of the acoustic meatus?
It is positioned obliquely: it forms an obtuse angle with the superior wall of the external acoustic meatus and an acute angle with the inferior wall.
What else to study on this topic
- Topographic anatomy of the middle ear: tympanic cavity and auditory tube
- Structure of the internal ear: osseous and membranous labyrinths
- Surgical anatomy of the tympanic cavity walls
- Syntopy of the temporomandibular joint and the external ear
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