Anatomy of the Ear





Introduction



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  1. The temporal bone forms part of the side and base of the skull. It constitutes two-thirds of the floor of the middle cranial fossa and one-third of the floor of the posterior fossa. There are four parts to the temporal bone:




    1. Squamosa



    2. Mastoid



    3. Petrous



    4. Tympanic



  2. The following muscles are attached to the mastoid process:




    1. Sternocleidomastoid



    2. Splenius capitis



    3. Longissimus capitis



    4. Digastric



    5. Anterior, superior, posterior, auricular (The temporalis muscle attaches to the squamosa portion of the temporal bone and not to the mastoid process.)



  3. The auricle (Figure 13-1) is made of elastic cartilage, the cartilaginous canal of fibro-cartilage. The cartilaginous canal constitutes one-third of the external auditory canal (whereas the eustachian tube is two-thirds cartilaginous), the remaining two-thirds is osseous. Innervation of auricle is outlined in Figure 13-2.



  4. The skin over the cartilaginous canal has sebaceous glands, ceruminous glands, and hair follicles. The skin over the bony canal is tight and has no subcutaneous tissue except periosteum.



  5. Boundaries of the external auditory canal are:




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    Anterior Mandibular fossa
    Parotid
    Posterior Mastoid
    Superior Epitympanic recess (medially)
    Cranial cavity (laterally)
    Inferior Parotid




    The anterior portion, floor, and part of the posterior portion of the bony canal are formed by the tympanic part of the temporal bone. The rest of the posterior canal and the roof are formed by the squamosa.



  6. Boundaries of the epitympanum are:




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    Medial Lateral semicircular canal and VII nerve
    Superior Tegmen
    Anterior Zygomatic arch
    Lateral Squamosa (scutum)
    Inferior Fossa incudis
    Posterior Aditus





  7. Boundaries of the tympanic cavity are:




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    Roof Tegmen
    Floor Jugular wall and styloid prominence
    Posterior Mastoid, stapedius, pyramidal prominence
    Anterior Carotid wall, eustachian tube, tensor tympani
    Medial Labyrinthine wall
    Lateral Tympanic membrane, scutum (laterosuperior)





  8. The auricle is attached to the head by




    1. Skin



    2. An extension of cartilage to the external auditory canal cartilage



    3. Ligaments




      1. Anterior ligament (zygoma to helix and tragus)



      2. Superior ligament (external auditory canal to the spine of the helix)



      3. Posterior ligament (mastoid to concha)



    4. Muscles




      1. Anterior auricular muscle



      2. Superior auricular muscle



      3. Posterior auricular muscle



  9. Notch of Rivinus is the notch on the squamosa, medial to which lies Shrapnell membrane. The tympanic ring is not a complete ring, with the dehiscence superiorly.



  10. Meckel cave is the concavity on the superior portion of the temporal bone in which the gasserian ganglion (V) is located.



  11. Dorello canal is between the petrous tip and the sphenoid bone. It is the groove for the VI nerve. Gradenigo syndrome, which is secondary to petrositis with involvement of the VI nerve, is characterized by:




    1. Pain behind the eye



    2. Diplopia



    3. Aural discharge



  12. The suprameatal triangle of Macewen triangle is posterior and superior to the external auditory canal. It is bound at the meatus by the spine of Henle, otherwise called the suprameatal spine. This triangle approximates the position of the antrum medially. Tegmen mastoideum is the thin plate over the antrum.



  13. Trautmann triangle is demarcated by the bony labyrinth, the sigmoid sinus, and the superior petrosal sinus or dura.


    Citelli angle is the sinodural angle. It is located between the sigmoid sinus and the middle fossa dura plate. Others consider the superior side of Trautmann triangle to be Citelli angle.


    Solid angle is the angle formed by the three semicircular canals.


    Scutum is the thin plate of bone that constitutes the lateral wall of the epitympanum. It is part of the squamosa.


    Mandibular fossa is bound by the zygomatic, squamosa, and tympanic bones.


    Huguier canal transmits the chorda tympani out of the temporal bone anteriorly. It is situated lateral to the roof of the protympanum.


    Huschke foramen is located on the anterior tympanic plate along a nonossified portion of the plate. It is near the fissures of Santorini.


    Porus acusticus is the “mouth” of the internal auditory canal. The canal is divided horizontally by the crista falciformis.



  14. There are three parts to the inner ear (Figure 13-3).




    1. Pars superior: vestibular labyrinth (utricle and semicircular canals)



    2. Pars inferior: cochlea and saccule



    3. Endolymphatic sac and duct



  15. There are four small outpocketings from the perilymph space:




    1. Along the endolymphatic duct



    2. Fissula ante fenestram



    3. Fossula post fenestram



    4. Periotic duct



  16. There are four openings into the temporal bone:




    1. Internal auditory canal



    2. Vestibular aqueduct



    3. Cochlear aqueduct



    4. Subarcuate fossa



  17. The ponticulum is the ridge of bone between the oval window niche and the sinus tympani.



  18. The subiculum is a ridge of bone between the round window niche and the sinus tympani.



  19. Körner septum separates the squamosa from the petrous air cells.



  20. Only one-third of the population has a pneumatized petrous portion of the temporal bone.



  21. Scala communis is where the scala tympani joins the scala vestibuli. The helicotrema is at the apex of the cochlea where the two join (Figure 13-4).



  22. The petrous pyramid is the strongest bone in the body.



  23. The upper limit of the internal auditory canal diameter is 8 mm.



  24. The cochlear aqueduct is a bony channel connecting the scala tympani of the basal turn with the subarachnoid space of the posterior cranial cavity. The average adult cochlear aqueduct is 6.2-mm long.





Figure 13-1


Auricle.






Figure 13-2


Sensory innervation of the auricle. C3, via greater auricular nerve; C2,3, via lesser occipital nerve; X, auricular branch; V3, auriculotemporal nerve; VII, sensory twigs.






Figure 13-3


Membranous labyrinth. A, ampulated end; NA, nonampulated end.






Figure 13-4


Organ of Corti.






Middle Ear



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  • Tympanic plexus = V3, IX, and X



  • V3 → Auriculotemporal nerve



  • IX → Jacobson nerve



  • X → Auricular nerve





Inner Ear



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See Figure 13-5.





Figure 13-5


Cross-section of internal auditory canal.






Blood Supply



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Tympanic Membrane



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See Figure 13-6.




Figure 13-6


Measurements of tympanic membrane.





The tympanic membrane has four layers:





  • (A) Squamous epithelium



  • (B) Radiating fibrous layer



  • (C) Circular fibrous layer



  • (D) Mucosal layer




Average total area of tympanic membrane: 70 to 80 mm2



Average vibrating surface of tympanic membrane: 55 mm2




Venous Drainage



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Ossicles



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Malleus





  1. Head



  2. Neck



  3. Manubrium



  4. Anterior process



  5. Lateral or short process




Incus





  1. Body



  2. Short process



  3. Long process (lenticular process)


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Apr 30, 2020 | Posted by in OTOLARYNGOLOGY | Comments Off on Anatomy of the Ear

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