The EAR Anatomy!

 This blog is all about the anatomy of the auditory system - OUTER and MIDDLE EAR!

The auditory system is divided into three main sections:

Outer ear;

Middle ear;

Inner ear.

Grossly, the outer ear comprises the auricle and ear canal. The middle ear, an air-filled cavity comprises the tympanic membrane, the three tiny ossicles, and the eustachian tube. The inner ear holds the sensory organs of hearing and balance.

Most of the structures that make up the ear is present within the major portions of the temporal bone.

The structures of the auditory conductive system are the outer and the middle ear, as they serve to conduct the acoustic signal to the cochlea and cochlear branch of the eighth nerve.

OUTER EAR:

        The externally visible aspect of the ear is an odd-shaped appendage called the pinna or auricle, which is usually what most people think of when a person says the word 'ear'. The entire framework of the pinna is composed of cartilage, except that of the earlobe. There are certain undifferentiated intrinsic as well as a number of extrinsic muscles that are present, which are vestigial in nature, in the case of humans. 

The pinna is built up of the skeleton of elastic cartilage and fibrous tissue, which is covered by a layer of skin on either side. This cartilaginous structure is continuous along with the external auditory canal/ external auditory meatus. It has an external and inner or cranial surface, wherein the former is faced laterally and the latter lies against the sides of the head. While the skin of the external surface is continuous with the skin covering the external auditory meatus and a part of its proceeds forward to become continuous over the skin that covers the parotid gland, the skin lining the inner surface proceeds backward along the skin that covers the head behind the ear.

LANDMARKS OF AURICLE:

            Since the cartilage of the auricle is curved by itself it has a number of elevations and depressions on the external and cranial surface. The auricle consists of an irregularly concave lateral surface and a convex medial surface. The superior and posterior margin forms a ridged rim along most of the perimeter of the pinna, the helix. The helix courses anteriorly and inferiorly, making a hook-like turn to form a shelf above the concha, which is the crus of the helix. The free margin of the helix presents a projection, the auricular tubercle (tuberculum auriculae). The deepest part lying at the center is known as the concha auriculae. This region is divided by the crus of the helix, into the cymba concha (superior part) and the larger part cavum concha (inferior part). The cavum concha is bounded by a backward folding cartilaginous ridge known as tragus anteriorly, which in turn turns posteriorly over the entrance to the ear canal. The posterior and inferior boundary of the cavum concha is the antitragus, lying opposite and inferior to the tragus. These two structures (tragus and antitragus) are separated by a deep notch known as the intertagic incisure or the intertragal notch. The concha is posteriorly bounded by a prominent semicircular ridge called the antihelix. The antihelix is divided superiorly into 2 ridges known as the crura of the antihelix. The bifurcations are wide superoposterior crus and narrower anterior crus, which end under the angle where the helix curves backward. A shallow depression is seen in between these two crura which is called the triangular fossa. Similarly, an elongated furrow that courses down the pinna separating the helix and antihelix is viewed and is termed the scaphoid fossa. The pinna ends inferiorly at the lobule or the ear lobe which is the only part of the pinna that is non-cartilaginous. The ear lobe is composed of soft fibro-fatty tissue. The depressions in the lateral surface are represented by the complementary elevations in the medial surface and vice versa. The elevations are namely the eminence of the concha, the eminence of triangular fossa, and the eminence of scapha; the depressions are the sulcus of the crus of the helix and the fossa of the antihelix. Darwin’s tubercle is a cartilaginous protuberance that presents as a thickening on the helix. The tubercle is dominantly inherited but still may or may not be visible.

Ligaments and Muscles of Pinna:

            The auricle is attached to the head by the continuous cartilage with the external auditory canal and through 3 specific extrinsic muscles and 3 extrinsic ligaments. Apart from this, 6 intrinsic muscles are also distinguished but are poorly represented in humans. The auricular muscles are regarded as vestigial in humans, whereas non-vestigial in lower animals.

Extrinsic Auricular Muscles:   

                                

Muscle	Origin	Insertion	Action
Anterior auricular muscle	Anterior region of the fascia of temporal region spine	Spine of the helix	Displacing the auricle anterosuperiorly
Superior auricular muscle	Fascia of temporal region upper	Upper part of the cranial surface (of auricle)	Displacing the auricle superior posterior
Posterior auricular muscle	Mastoid bony cranial	Cranial surface (at the convexity of concha)	Displacing the auricle backward

                    

Intrinsic Auricular Muscles

            There are six intrinsic auricular muscles out of which four lie on the lateral surface and two on the cranial surface of the pinna.

Muscles on Lateral surface muscles	Muscles on Cranial surface
Helicis major          Helices minor         Tragicus         Antitragicus	Transversus auricular            Obliquus auricular

1. Helicis major originates from the crus of the helix, stretches up to, and gets inserted into the anterosuperior margin of the helix.

2. Helicis minor covers the crus of the helix.

3. Tragicus runs on the lateral aspect of the tragus. 

4. Antitragicus stretches from the outer part of antitragus to the antihelix and tail of the helix.

5. Transversus auriculae extend between eminence of concha and eminence of scapha. 

6. Obliquus auriculae extends between the eminence of the concha and the eminence of the triangular fossa.

LIGAMENTS:

Extrinsic Ligaments:

            The cartilage of pinna is attached to the temporal bone by the anterior ligament and posterior ligament. The anterior ligament connects the tragus and anterior rim of the crus of the helix to the root of the zygomatic arch. The medial surface of the concha is adhered to the lateral surface of the mastoid prominence by the posterior ligament.

Intrinsic Ligaments:

            These connect parts of cartilage within the auricle. One of the ligaments is a strong, fibrous band that runs between the tragus and helix and the other extends between the antihelix and the inferior portion of the helix.

BLOOD SUPPLY TO PINNA:

            The arterial supply arises predominantly from the branches of the external carotid artery, the superficial temporal artery, occipital artery, and posterior auricular artery.

            Venous drainage contains veins that drain into the external jugular vein. A superficial temporal vein takes away blood from the lateral surface of the auricle. The posterior auricular vein takes away blood from the cranial surface of the auricle.

NERVE SUPPLY TO AURICLE:

                    The superficial surface of the pinna is innervated by the great auricular nerve and lesser occipital nerve from the cervical plexus and also innervated by the auriculotemporal division of the trigeminal mandibular nerve. The deepest parts are innervated by the auricular branch of the vagus and facial nerve which sends a branch to the auricular branch of the vagus nerve.

REFERENCES

1. Singh, I. (2008). Textbook of Anatomy with Colour Atlas (4th ed.). Macmillan Publishers

2. Anson, B. J. (1973). Surgical anatomy of the temporal bone and ear (2nd ed.). Saunders.

3. Gelfand, S. A. (2009). Hearing: An Introduction to Psychological and Physiological Acoustics. Switzerland: CRC Press.

4. Singh, D. R. (2017). Essentials of Anatomy for Dentistry Students. Wolters Kluwer.

5. Janfaza, P., & D, P. J. M. (2011). Surgical Anatomy of the Head and Neck. Amsterdam University Press.

6. Singh, V. (n.d.). Textbook of Anatomy: Head, Neck and Brain, Vol 3, 3rd Updated Edition, EBook. India: Elsevier Health Sciences.

7. Drake, R., Mitchell, A. W. M., Vogl, A. W. (2009). Gray's Anatomy for Students - Rental. United Kingdom: Elsevier Health Sciences.

EXTERNAL AUDITORY CANAL

  

                    Also called external auditory meatus or ear canal, serves as communication between the external environment and the middle ear. It is an S-shaped tube that is 2.5-3.5cm long with a diameter of 0.7cm and a height of 0.9cm. It moves from the concha anterior superiorly, posterior superiorly, and then anterior inferiorly to the tympanic membrane. The diameter is largest at the auricular orifice, becoming gradually smaller toward the isthmus, which is the junction of the cartilaginous framework with the bony framework. The diameter expands again only to decrease in size just before the meatus terminates medially at the tympanic membrane. The outer 1/3rd of the ear canal is cartilaginous and the inner 2/3rd is bony. The diameter of the bony portion is fixed while that of the cartilaginous portion varies and its diameter is dependent on such things as the movement of the mandible. At birth, there is no osseous canal and it does not become fully developed until the end of the third year. The bony canal develops from the tympanic annulus. In children up to 4-5 years of age, there is a gap in the anteroinferior wall of the osseous part known as the foramen of Huschke. The anterior, inferior, and lower posterior parts of the bony wall are formed from the Tympanic part, Squamous and Petrous part of the temporal bone and these walls are closely related to the parotid gland. The anterior wall of the bony meatus is also closely related to the glenoid fossa and condyle of the mandible.

ORIENTATION OF THE MEATUS:

Adult - S-shaped tube: Anterior superiorly, posterior superiorly, and then anterior inferiorly.

Children -  It is more horizontal.

BLOOD SUPPLY:

                    The anterior wall of the ear canal is supplied by branches from the superficial temporal artery (from the external carotid). The posterior wall is supplied by branches from the posterior auricular artery (from the external carotid). The inner part of the meatus is supplied by the deep auricular artery (branch of the internal maxillary artery) which enters between the cartilaginous and bony parts.

Venous drainage of the anterior part of the ear canal is through the retro mandibular vein; the posterior part by the posterior auricular vein.

NERVE SUPPLY:

           The anterior wall of the ear canal is innervated by the auriculotemporal nerve and the posterior wall of the ear canal is innervated by the auricular branch of cranial nerve X with contributions from cranial nerves VII and IX.

REFERENCES

1. Beagley, H.A.(1981). Audiology and Audiological Medicine. Oxford: Oxford University Press

2. Zemlin, W.R. (2011). Speech and Hearing Science; Anatomy and Physiology (4th edition). Allyn and Bacon: Boston

 

TYMPANIC MEMBRANE

                    The external auditory meatus ends at the tympanic membrane or the eardrum, which is tilted at an angle of ~ 55° to the canal. The tympanic membrane is firmly attached to the tympanic sulcus, a groove in the bony canal wall, by a ring of fibro cartilaginous connective tissue called the tympanic annulus or annular ligament. The ring has a disruption in the tympanic sulcus known as the notch of Rivinus.

           The tympanic membrane is a translucent membrane that is pinkish grey in color with an average thickness of   ~ 0.074 mm and has a vertical diameter of ~ 0.9 to 1.0 cm and a horizontal diameter of ~0.8 to 0.9 cm. TM is concave outwards and the Central portion where the handle of the malleus is attached is called the umbo.

           The lateral part of the malleus called the manubrium attaches almost vertically to the eardrum, with its tip at the umbo. The attachment of the manubrium of the malleus forms the malleal prominence. The ligamentous bands called the anterior and posterior malleal folds run from both sides of malleal prominence to the notch of Rivinus.

TM is mainly divided into 4 quadrants: 

anterior superior, anterior inferior, posterior superior, and posterior inferior.

            The presence of a cone of light is considered the hallmark of a healthy eardrum. The light reflection is seen as a right area on the anterior inferior surface of the eardrum, radiating from the tip of the manubrium to the 5’o clock position in the right ear and the 7’o clock position in the left ear. 

TYMPANIC MEMBRANE LAYERS:

                    The most lateral layer of TM is continuous with the skin of the ear canal, and the most medial layer is continuous with the mucous membrane of the middle ear, in between these 2 layers is the fibrous layer that is composed of radial fibers and the concentric circular fibers.

           Outer epithelial layer: continues with the lining of EAM

        Middle fibrous layer (supports the TM): consists of radial (radiate from the handle of malleus) and concentric circular fibers (radiate from the periphery)

          Inner mucous layer: continues with the lining of the tympanic cavity.  

The TM is divided into 2 parts by the anterior and posterior malleal folds called the pars flaccida (also called the Shrapnell’s membrane) and pars tensa also called the tense part because it contains all the 3 layers that are the lateral layer, medial layer, and the fibrous layers but the pars flaccida lacks the fibrous layer. 

BLOOD SUPPLY:

                    The external surface of TM is supplied by the deep auricular branch of the maxillary artery. The internal surface of the tympanic membrane is supplied posteriorly by the stylomastoid branch of the posterior auricular artery, and anteriorly by the tympanic branch of the maxillary artery.

NERVE SUPPLY:

                   The lateral surface- anterior part of the tympanic membrane receives sensory innervation from the auriculotemporal branch of the mandibular nerve, a branch of the trigeminal nerve (V3),

The lateral surface- posterior part of the tympanic membrane receives sensory innervation of the auricular branch of the facial nerve (CN VII), the auricular branch (Arnold's nerve )of the vagus nerve (CN X), and the glossopharyngeal nerve (CN IX).

The medial surface of the tympanic membrane receives sensory innervation from the glossopharyngeal nerve's tympanic branch (Jacobson's nerve) (CN IX).

REFERENCES

1. Gelfand, S. A. (2004). Hearing: Introduction to Psychological and Physiological Acoustics. (4^th Edn.). New York: Marcel Decker.

2. Zemlin, W. R. (1998). Speech & Hearing Science: Anatomy & Physiology. Boston: Allyn & Bacon

MIDDLE EAR CAVITY

                    The tympanic cavity is an air-filled compartment surrounded by bone that is separated from the external ear by a thin tympanic membrane and is in direct communication with the pharynx via the auditory tube. The tympanic cavity is divided into five parts: 

Pro-tympanum: The pro-tympanum includes a part of the osseous eustachian tube, the semi-canal of the tensor tympani muscle, and the supratubal recess, and the cochleariform process with the tensor tympani tendon. It covers the internal carotid artery, the cochlea, the geniculate ganglion, and the petrous portion of the facial nerve. There is a clear borderline between the pro-/mesotympanum and the epitympanum, called the tympanic diaphragm. The anterior margin of the pro-tympanum is the aperture of the eustachian tube, and its posterior margin is the line projected from it onto the promontory of the anterior margin of the tympanic annulus.

Meso-tympanum: The mesotympanum is circumferentially defined by the annulus and the lateral process of the malleus and contains the stapes superstructure, incus long process, malleus handle/umbo, tympanic facial nerve, oval and round windows. The lateral boundary of the mesotympanum is the pars tensa of the tympanic membrane while the cochlear promontory serves as the medial boundary.

Retro-tympanum: The retrotympanum extends from the posterior annulus to the anterior border of the mastoid facial nerve and contains several spaces including the sub pyramidal space, sinus tympani, and sinus sub tympanicum that often house residual cholesteatoma disease. The retrotympanum is separated from the hypotympanum by the funiculus, which is a bridge of bone that extends over the sub-cochlear canaliculus and contains Jacobson’s nerve. The sub-cochlear canaliculus is a natural pathway medially that communicates with variable present air cells in the inferior petrous apex.

Hypo-tympanum:  The medial border of the hypotympanum is the bone covering the posteriorly located jugular bulb and the anteriorly located vertical petrous carotid artery. The superior border of the hypotympanum laterally is the inferior annulus. The protiniculum is a variable crest of bone the separates the hypotympanum from the pro-tympanum. The posterior border of the pro-tympanum is the anterior annulus and contains the semi-canal for the tensor tympani muscle, horizontal petrous carotid artery, and osseous orifice of the eustachian tube.

Epi-tympanum:  The epi-tympanum is defined as the space between the tegmen tympanum (roof of the middle ear) and the lateral process of the malleus with the lateral wall being comprised of the scutum and pars flaccid, and the medial wall being the bone covering the geniculate ganglion, and ampullated ends of the lateral and superior semi-circular canals. The posterior aspect of the epi-tympanum connects to the mastoid air cell system via the aditus ad antrum. The incus body and head and neck of the malleus are suspended in the central aspect of the epitympanum by the incudal ligament and the anterior and posterior malleolar ligaments respectfully.

WALLS OF THE TYMPANIC CAVITY

                    The tympanic cavity is ‘bounded by six not too clearly demarcated walls in each direction.

    ROOF: The superior paries tegmentalis (tegmen tympani) separates the tympanic cavity from the middle cranial fossa, forming the roof of one and the floor of the other compartment. The tegmen tympani is often composed of only a very thin bony layer, thus posing the risk of dura injury when manipulating in this area, or spread of a middle ear infection to the intracranial space. This bone continues in a posterior direction to form the roof of the tympanic atrium, where there is an indirect communication between the middle ear space and the mastoid air cells.

    FLOOR: The inferior paries jugularis represents the floor of the tympanic cavity (hypotympanum). Normally, this floor consists of bone but it might also be irregularly excavated by some air-filled cells. Right underneath the floor, the jugular bulb (fossa of the internal jugular vein) lingers and can be inadvertently opened by the surgeon, possibly leading to the heavy and troublesome venous bleeding referred to earlier. At the junction of the floor and the medial wall of the middle ear, there is a small opening that allows the entry of the tympanic branch of the glossopharyngeal nerve to pass into the middle ear. This nerve takes an important part in the formation of the tympanic plexus.

    MEDIAL WALL: The medial paries labyrinth is mostly represented by the labyrinth capsule, i.e. the lateral cochlear wall with its basal turn forming the promontory here. The oval window (vestibular window; fenestra vestibuli) lies dorso-superior to the promontory with the round window (fenestra cochleae) caudal to it (Lang, 1992). While the oval window is closed by the stapes footplate, the round window is sealed only by a membrane. The medial wall of the cavity is formed by a dense portion of the temporal bone, which houses the inner ear. Significant landmarks on this wall include the promontory (a rounded prominence formed by the lateral projection of the basal turn of the cochlea), the round window (an opening into the basal turn of the scala tympani), the oval window (an opening into the vestibule of the inner ear that accommodates one of the middle ear bones, the stapes), and the prominence of the facial nerve through which the facial nerve courses.

    LATERAL WALL: The lateral paries membranaceus is essentially formed by the TM. However, a portion of the lateral wall above the TM (in the epitympanic recess) is formed by a section of the squamous portion of the temporal bone.

    POSTERIOR WALL: The posterior border (paries mastoidea) is indistinctly circumscribed by the sinus tympani. Its superior part opens into the mastoid via the antrum area. The posterior wall of the middle ear is wider above than below. In its upper part, it has an important opening known as the aditus. This aditus helps the middle ear communicate with the mastoid air cell system. Aditus is a large irregular opening connecting the mastoid antrum to the middle ear cavity. Below the aditus is a small depression known as the fossa incudis. Fossa incudis houses the short process of the incus. Below the fossa incudis lies the pyramid. Pyramid is a small conical projection that is hollow and its apex pointing anteriorly. It contains the stapedius muscle, the tendon of which passes forwards to insert into the neck of the stapes. The canal within the promontory curves downwards and backward to join the descending portion of the facial nerve canal. Between the promontory and the tympanic annulus is the facial recess. The facial recess is bounded medially by the facial nerve and laterally by the tympanic annulus. Running through the wall between the two with varying degrees of obliquity is the chorda tympani nerve. This nerve always runs medial to the tympanic membrane.

    ANTERIOR WALL: The anterior border is called the paries caroticus, a name signaling that this region allows only cautious manipulations, as the opening of an exposed carotid artery might have life-threatening consequences. Superior and lateral to the carotid artery, the Eustachian tube, which is lined by pseudostratified, ciliated columnar epithelium with scattered goblet cells, opens into the tympanic cavity. Medial and parallel to the tube, the tensor tympani muscle runs in its bony semi-canal. The anterior wall often referred to as the carotid wall, is a thin plate of bone that is perforated in its superior region by an opening for the auditory or Eustachian tube (ET) and an orifice for the tendon of the tensor tympani muscle (There is additionally one small aperture that opens into the posterior wall near the juncture of this wall with the lateral wall through which a branch of the facial nerve, the chorda tympani, enters the middle ear and courses through the middle ear to exit the cavity near the tympanic sulcus.

MASTOID ANTRUM:

                    It is one of the largest air cells in the temporal bone which anteriorly communicates with the attic via the aditus.

Dimension: 

§  Anteroposterior length- 10 mm

§  Vertical height- 10-15 mm

§  Side to side width- 5 mm

MASTOID AIR CELLS:

                    They are highly variable in their shape & number. These develop as buds from antrum, the formation of air cells being called pneumatization. The buds invade the mastoid part of the temporal bone from birth to puberty when the process is complete. When small, the mastoid part of temporal bone & the mastoid process are almost solid but as they enlarge marrow appears in the interior. Normally the marrow is later replaced by air cells which invade it from above downward.  Small air cells are usually found in the posterior wall. Air cells may also extend around the facial canal and around the semi-circular canals. The aditus is seen in the upper third of the wall. It is a large irregular aperture, which leads backward from the epitympanic recess into considerable air space, named the tympanic or mastoid antrum incudis. Just below this is the facial canal which descends downwards vertically in the petrous part of the temporal bone to reach the stylomastoid foramen. On the facial canal is a small bony projection called the pyramidal eminence. It is hollow & from its cavity wall, the stapedius muscle arises and then passes through a foramen at its apex from which it runs into its insertion into the neck of the stapes. At birth, the mastoid is not pneumatised but becomes aerated over the first year of life. Poor pneumatization is associated with ET dysfunction.

The mastoid air cell system is categorized according to various regions of the temporal bone. These include:

a. Squamo mastoid – This area includes air cells around the antrum, central mastoid tract, and peripheral air cell tract.

b. Perilabyrinthine cells – These can be divided into supra labyrinthine and infralabyrinthine air cells

c. Petrosal air cells – Petrosal air cells and petrous apex air cells

d. Accessory air cells – These cells include zygomatic air cells, occipital air cells, squamous air cells, and styloid air cells.

OSSICLES

                    Surrounded the middle ear cavity is a series of three bones in the ossicular chain, that forms a bridge between the tympanic membrane and the inner ear. The Malleus, Incus, and Stapes are the three smallest bones in the human body and are referred to as the Ossicular chain. They reach their adult size late during the fetal period, and the overall shape and size do change substantially following birth.

MALLEUS

                    The first of the three bones in the ossicular chain. It is about 9 mm overall in length and weighs around 23 to 37 mg. The bone consists of the head, neck, and three processes. 

The manubrium, one of the three processes, attaches firmly to the tympanic membrane, in the middle of the eardrum. The head of the malleus large bulb-shaped portion of the ossicle that projects in an upward direction from the manubrium into the epitympanic recess. The posterior surface of the bone contains an articular facet, that serves as the point of connection between the malleus and incus. The neck of the malleus is a constriction that is located between the manubrium and head. At the point where the manubrium joins the neck, there is a small projection that forms the point of attachment for the tensor tympani muscle. The anterior process is a spine-like process that can be seen in the region of the juncture of the manubrium and the head. In this area is the lateral process, which is directed laterally and attaches to the upper portion of the tympanic membrane.

INCUS

                    The middle one of the three bones in the ossicular chain. The weight of the incus is around 23 - 32 mg. and the short process measures an average of 5 mm in length whereas the long process is around 7 mm in length. Consists of three parts: a body, and two crura or processes.

On the anterior surface of the body is an articular facet that serves as the contact point for the incus connection to the malleus. The two processes of the incus arise from the body of this bone at right angles to each other. The short process is directed backward in a horizontal plane and shares the epitympanic recess with the head of the malleus. The long process courses in a vertical direction parallel to the manubrium. At its inferior end, the long process turns abruptly in a medial direction and terminates as a rounded projection called the lenticular process. The medial end of the process is covered with cartilage and articulates with the head of the stapes.

STAPES

                    The third bone and the smallest bone of the human body is the stapes. The height of the stapes is around 2.5 to 3.8 mm and weighs around 2.1 to 4.3 mg. It consists of a head, neck, two crura, and a footplate.

The head of the stapes contains a concave articular that forms the point of connection for the lenticular process of the incus. The head has a small spine where the tendon of the stapedius muscle attaches. Medially from the neck, the two crura connect the footplate to that of the neck of the stapes. The anterior crus is shorter and slender than the posterior crus. The medial surface and the periphery of the footplate are covered by a thin layer of hyaline cartilage which is fastened to the bony wall of the oval window by the annular ligament.

SUPPORT FOR THE OSSICULAR CHAIN

                     The ossicular chain is suspended in the middle ear cavity by the tendons from two muscles and several ligaments. The tendons of the two muscles are the tensor tympani muscle and the stapedius muscle. 

The tendon of tensor tympani enters the middle ear space through the anterior wall of the middle ear cavity and inserts into the malleus at the point where the manubrium meets the neck. It is innervated by the fifth cranial nerve, which when innervated pulls the malleus in an anterior and posterior direction. 

The second muscle is the stapedius muscle, which originates in a bony canal behind the middle ear space. It enters the cavity through a small opening on the posterior wall of the cavity called pyramidal eminence. Attaches to the head of stapes and is innervated by the seventh cranial nerve. When this muscle contracts, it pulls the stapes in a posterior direction.

EUSTACHIAN TUBE

                    The eustachian tube runs from the anterior wall of the middle ear cavity to the posterior wall of the nasopharynx in an inferior, medial and anterior direction at an angle of approximately 45 degrees in the adult. The first third of the tube, around 12 mm has a bony foundation, while the remaining portions are cartilaginous in nature (18-24 mm).  The isthmus is the point where both the bony and the cartilaginous portions meet, which is a narrow point compared to the remainder of the tube. The lumen through the bony portion is normally open, or patent and varies from 3-6 mm in diameter. At rest, the cartilaginous part of the tube is closed, but it opens on the action of the levator veli palatini. The muscles of the eustachian tube are four in number. They are tensor veli palatini (TVP), levator veli palatini (LVP), salpingopharyngeus, and the tensor tympani muscle. The ostmann fat pad is located in the inferolateral aspect of the ET and is thought to be an important contributing factor in closing the tube.

                    

VASCULAR SUPPLY TO THE MIDDLE EAR

BRANCH	PARENT ARTERY	REGION SUPPLIED
Anterior tympanic	Maxillary artery	Malleus, Incus, TM, Part of tympanic cavity
Stylo mastoid	Posterior auricular	Posterior part of the tympanic              membrane, Stapedius muscle
Petrosal	Middle meningeal	Roof of mastoid& epitympanum
Superior tympanic	Middle meningeal	Malleus, incus, tensor tympani
Inferior tympanic	Ascending pharyngeal	Mesotympanum
Branch from the parent artery	Artery of pterygoid canal	Meso & hypotympanum
Caroticotympanic branch	Internal carotid	Meso & hypo tympanum

LYMPHATIC DRAINAGE OF THE MIDDLE EAR

Anterior lymphoid drainage comes to pre-auricular lymph nodes.

Posterior lymphoid drainage comes to retropharyngeal lymph nodes

NERVE SUPPLY

                    The nerve comprises the tympanic plexus, which ramify upon the surface of the promontory. 

The plexus is derived from

    The tympanic branch of the glossopharyngeal (Jacobson’s nerve) enters the tympanic cavity by an opening in its floor close to the labyrinthine wall. 

    The superior and inferior caroticotympanic nerves from the carotid plexus of the sympathetic pass via the wall of the carotid canal and join the branches of the tympanic branch of the glossopharyngeal. 

    The smaller superficial petrosal nerve enters the anterior surface of the petrous part of the temporal bone through a small aperture, situated lateral to the hiatus of the facial canal. It courses inferiorly, past the geniculate ganglion of the facial nerve, receiving a connecting filament from it, and enters the tympanic cavity, where it communicates with the tympanic branch of the glossopharyngeal, and assists in forming the tympanic plexus.

    The branch to the greater superficial petrosal passes through an opening on the labyrinthine wall, in front of the fenestra vestibuli.

REFERENCE

 

1. Gelfand, S. A. ESSENTIALS OF AUDIOLOGY. NewYork: thieme.

2. Moller, A. R. HEARING: ANATOMY, PHYSIOLOGY, AND DISORDERS OF THE AUDITORY SYSTEM second edition. Sydney: Elsevier.

3. Zemlin, W. R. (1997). W.R. Zemlin’s Speech and Hearing Science Speech and Hearing Science: Anatomy and Physiology (4th Edition) [Hardcover](1997) (4 edition). Allyn & Bacon.)

4. Anthwal N, Thompson H. The development of the mammalian outer and middle ear. J Anat. 2016 Feb;228(2):217-32.)