Anatomy and Embryology of the Ear

Anatomy of the Ear

•  The ear is divided into three parts: external, middle, and inner.
• The external ear is primarily concerned with sound transmission.
• The middle ear transmits sound.
• The inner ear functions as the organ for sound.

Embryology of the Ear

• The auricle develops from a series of six tubercles.
• The auricle forms around the margins of the first visceral cleft.
• This occurs around six weeks of gestation.
• The external auditory canal is formed from the ectoderm of the first visceral cleft.

Tympanic Membrane

• The tympanic membrane has three layers: an outer epithelial layer (ectoderm), a middle fibrous layer (mesoderm), and an inner mucosal layer (endoderm).
• The outer epithelial layer comes from the first visceral cleft.
• The fibrous layer lies between the first visceral cleft and tubotympanic recess.
• The inner mucosal layer is from the endoderm of the tubotympanic recess.

External Ear

• The external ear includes the auricle (pinna) and the external auditory meatus.
• The auricle consists of yellow elastic cartilage (except the lobule, which is composed of fat).
• The auricle is covered with skin and attached to the head by ligaments.
• The external auditory meatus measures 24mm in adults. It has two parts: a cartilaginous part of 8mm and a bony part of 16mm.
• The sebaceous glands, ceruminous glands, and hair follicles are in the cartilaginous portion.
• The anatomy of the external ear can aid clinicians with the safety of piercing the lobule, avoiding cartilage.

Nerve Supply of External Ear

• Auriculotemporal nerve (V1)
• Facial nerve (VII)
• Auricular branch of the vagus (Arnold)
• Lesser occipital nerve (C2)
• Greater auricular nerve (C2C3)

Blood Supply of External Ear

• Superficial temporal artery
• Postauricular artery
• Maxillary artery

The Middle Ear

• The tympanic membrane separates the external auditory meatus from the middle ear.
• The tympanic membrane is nearly oval-shaped.
• The angle it forms with the floor of the meatus is approximately 55 degrees.
• The tympanic membrane has an outer layer of squamous epithelium, a middle fibrous tissue layer, and an inner layer of mucous membrane.
• The tympanic membrane's periphery is supported by a fibrous thickening known as the annulus.
• The annulus fits into a slot in the tympanic bone called the sulcus.
• Landmarks of the ear drum include the cone of light, the umbo, and the malleus handle.
• The tympanic membrane is divided into pars tensa and pars flaccida.
• The pars flaccida, also known as the attic, is where the fibrous layer is absent.

Landmarks of the Ear Drum

• The ear drum has a pearly grey color.
• It has a triangular bright area, cone of light.
• The cone of light extends from the center (umbo) downwards and forwards.
• The most prominent landmark is the handle of the malleus, at its upper end, a small projection called the umbo.
• Chronic perforations of the attic are potentially unsafe.

Nerve Supply of the Middle Ear

• The outer surface of the tympanic membrane is supplied similarly to the external auditory meatus.
• The inner surface of the tympanic membrane is supplied by the tympanic branch of the 9th nerve (Jacobson nerve).
• Blood supply to the middle ear is by the postauricular and maxillary arteries.

Middle Ear Cleft

• Includes the Eustachian tube, the middle ear cavity, and the aditus to the mastoid antrum and air cells.

Eustachian Tube

• Connects the tympanic cavity to the nasopharynx.
• It's shorter, wider, and more horizontal in infants.
• It opens on yawning or swallowing.
• This action is due to the contraction of the tensor palati muscle.
• Helps maintain equal air pressure on both sides of the tympanic membrane.

Middle Ear Cavity

• Described as a six-sided box.
• Often shaped like a matchbox.

Walls of the Middle Ear

• Roof (tegmen tympani): Separates from the tympanic cavity and mastoid antrum from the middle cranial fossa.
• Floor: Separates the cavity from the bulb of the internal jugular vein.
• Posterior wall: Contains the aditus, connecting the tympanic cavity to the mastoid antrum. Just below is the pyramid, housing the stapedius muscle.
• Anterior wall: Separates from the internal carotid artery. Also contains the Eustachian tube.
• Lateral wall: The tympanic membrane.
• Medial wall: Contains the promontory, covering the basal turn of the cochlea. The facial nerve also runs horizontally superior to it and is enclosed in the canal of Fallopius. The round window, and the oval window are in this wall.

Inner Ear

•  Lies within the petrous part of the temporal bone.
•  The inner ear is also called the labyrinth.
•  The structure is complex and consists of a membranous labyrinth enclosed in a bony labyrinth (otic capsule).
• The membranous labyrinth contains fluid called endolymph (similar to intracellular fluid) and the space between the bony labyrinth and membranous labyrinth is filled with perilymph (similar to extracellular fluid).

Bony Labyrinth

• Cochlea: A snail-like structure (anterior).
• Vestibule: Located in the middle.
• Contains the oval window (fenestra ovale), closed by the stapes' footplate.
• Semicircular canals: Arranged posteriorly in three planes.

Membranous Labyrinth

• Cochlear duct: Located within the bony cochlea, containing neuroepithelium (organ of Corti).
• Nerve fibres pass to the cochlear nerve.
• Saccule and utricle: Located in the vestibule, sensitive to linear acceleration.
• Semicircular canals: Each set at right angles to each other. They are sensitive to angular acceleration.
• The SCC, utricular, and saccular nerves form the vestibular nerve.

Blood Supply to theInner Ear

• The labyrinthine artery (internal acoustic artery) supplies blood to this part.
• It branches from the basilar or anterior inferior cerebellar artery.

The Facial Nerve

• Begins at the pons, crosses the cerebellopontine angle, and enters the temporal bone via the internal auditory meatus.
• It passes over the labyrinth then reaches the medial wall of the tympanic cavity.
• It then bends at a right angle, encased in the Fallopian canal, and travels above the oval window.
• Reaching the aditus, it turns downwards behind the pyramid and emerges through the stylomastoid foramen.
• The nerve to the stapedius muscle branches off near the pyramid.
• The chorda tympani nerve leaves the facial nerve and enters the tympanic cavity.

Summary

• Familiarity with ear anatomy is fundamental to otological practice.

Questions?

Explore the intricate anatomy and development of the ear, including its three main parts: external, middle, and inner ear. This quiz delves into the embryological formation of the ear structures and the layers of the tympanic membrane. Test your knowledge on the ear's functions and developmental processes.