Anatomy and Physiology of the EarCHAPTER 1

Pinna develops from six tubercles around the first branchial cleft. Time of development is given in Table 1.1.

External auditory canal develops from the first branchial cleft.

Tympanic membrane develops from all the three germinal layers. Outer epithelial layer is formed by the ectoderm, inner mucosal layer by the endoderm and the middle fibrous layer by the mesoderm.

Middle ear cleft develops from the tubotympanic recess.

The malleus and incus develop from the first branchial arch.

4The stapes develops from the second branchial arch while the footplate is derived from the otic capsule.

Inner ear: The ectoderm in the region of hind brain thickens to form an auditory placode, which is invaginated to form auditory vesicle or the otocyst. It differentiates into the endolymphatic duct and sac, utricle, semicircular canals, saccule and cochlea.

Endural incision: It is made in incisura terminalis for the surgery of external auditory canal and middle ear. It does not cut through the auricular cartilage.

Grafts in tympanoplasty: Tragal and conchal cartilage and perichondrium and fat from lobule are often used during tympanoplasty surgery.

Grafts in rhinoplasty: The conchal cartilage is commonly used to correct depressed nasal bridge. The composite 5grafts of the skin and cartilage can be used for repair of defects of ala of nose.

Frost bite: The outer surface of pinna is more prone to frost bite because the skin is adherent to the underlying perichondrium. There is no subcutaneous tissue.

Sebaceous cysts: They are more common on medial surface of pinna.

The roof is formed by a thin plate of bone called tegmen tympani.

The floor is formed by a thin plate of bone, which separates tympanic cavity from the jugular bulb.

The anterior wall is formed by opening for canal of tensor tympani muscle above, below opening of Eustachian tube and a thin bony plate separating the tympanic cavity from internal carotid artery.

The posterior wall is related to pyramid, aditus, facial nerve and facial recess. Facial recess is a depression in the posterior wall lateral to pyramid and is bounded medially by the vertical part of facial nerve, laterally by chorda tympani nerve and above by the fossa incudis.

The medial wall is related to promontory (most prominent and bulging part of the medial wall formed by the basal turn of the cochlea), bony lateral semicircular canal, oval window (fenestra vestibule), round window (fenestra cochlea) and the facial nerve running in bony fallopian canal above the oval window.

The lateral wall is formed by the tympanic membrane and partly by a portion of squamous part of the temporal bone. This wall separates the middle ear from external ear.11

Ossicles (Fig. 1.5)

• Malleus: Hammer shaped, largest and most lateral ossicle measuring 8 mm in length. It has head, neck, handle and also anterior and lateral processes.
• Incus: Anvil shaped, has a body, short process and long process. The body articulates with the head of the malleus in the attic.
  

  Fig. 1.5: Middle ear
  12

  Table 1.4   The muscles of tympanic cavity Muscle Origin Insertion Innervation Function Tensor tympani Cartilaginous pharyngotympanic tube, greater wing of sphenoid, its own bony canal Upper part of handle of malleus Mandibular nerve (V3) Contraction pulls handle of malleus medially, tensing tympanic membrane to reduce the force of vibrations in response to loud noise Stapedius Attached to inside of pyramidal eminence Neck of stapes Branch of facial nerve Contraction, usually in response to loud noises, pulls the stapes posteriorly and prevents excessive osscillation

• Stapes: Stirrup shaped, smallest ossicle, measuring about 3.5 mm and consists of a head, neck, footplate and also anterior and posterior crura. The footplate is held to the oval window by the annular ligament.

Muscles: Tensor tympani and stapedius muscles (Table 1.4).

Ligaments keep the ossicles in their place.

Nerves: Chorda tympani nerve. It enters the middle ear from posterior wall, runs forwards and lateral to the incus and medial to the malleus, exit through the anterior wall.

Air: It fills the tympanic cavity and the mastoid.13

The internal ear or the labyrinth is the organ of hearing and balance (Figs 1.8 and 1.9).

It consists of bony and membranous labyrinth (Table 1.6).

The membranous labyrinth is filled with endolymph while the space between the membranous and bony labyrinth is filled with perilymph (Table 1.7).17

Bony labyrinth consists of three parts: the vestibule, the semicircular canals and the cochlea.

Table 1.6   Parts of the inner ear Parts of bony labyrinth Parts of membranous labyrinth Sensory end organs Exciting stimulus Nerve Semicircular canals Semicircular canals Crista in the ampula Angular acceleration Caloric stimulation Vestibular Vestibule Utricle Saccule Macula Gravitational pull Linear acceleration Sound Vestibular Cochlea Cochlear duct Organ of Corti Sound Cochlear

The vestibule is the central part of the bony labyrinth.

The oval window is present in the lateral wall of the vestibule.

The inside of the medial wall of the vestibule presents two recesses, a spherical recess, which lodges the saccule and an elliptical recess which possesses utricle. Below the elliptical recess, there is opening for aqueduct of vestibule through which passes the endolymphatic duct.

There are three semicircular canals, the lateral, posterior and superior, and lie in planes at right angles to one another.

Each semicircular canal has an ampullated end which opens independently into the vestibule and a non-ampullated end. The non-ampullated ends of posterior and superior canals unite to form a common opening called as the crus commune.19

The three semicircular canals open into the vestibule by 5 openings.

The bony cochlea is a coiled tube making 2.5–2.75 turns round a central pyramid of bone called the modiolus.

The bony cochlea contains three compartments—scala vestibuli, scala tympani and scala media (Fig. 1.10).

The scala vestibuli and scala tympani are filled with perilymph and communicate with each other at apex called helicotrema.

The scala tympani is closed by secondary tympanic membrane; it is also connected with the subarachnoid space through the aqueduct of cochlea.

The scala vestibuli is closed by footplate of stapes which separates it from middle ear.20

The membranous labyrinth consists of the cochlear duct, the utricle and saccule and the endolymphatic duct and sac.

The cochlear duct present in the bony cochlea around the modiolus. It is connected to the saccule by the ductus reunions. The cochlear duct is triangular in cross-section with Reissner's (vestibular) membrane, stria vascularis and the basilar membrane forming its three sides. The end organ in the cochlea is the neuroepithelium called the organ of Corti and it is spread like a ribbon along the entire basilar membrane. It is stimulated by sound. It consists of tunnel of Corti which is composed of two rows of rods of Corti. It forms triangle with basilar membrane and contains cortilymph. The tectorial membrane overhangs the organ of Corti. There are hair cells in the organ of Corti which are important receptor cells of hearing and transduce sound energy into electrical energy. Inner hair cells form a single row while outer hair cells are arranged in three or four rows. Inner hair cells are richly supplied by afferent cochlear fibers whereas the outer hair cells mainly receive efferent innervation from the olivary complex and are concerned with modulating the function of inner hair cells (Table 1.8). There are supporting cells like Deiters’ cells which present between the outer hair cells and provide support to latter. Cells of Hensen lie outside the Deiters’ cells.