Talk to us?

- NEETMDS- courses
NEET MDS Lessons
Anatomy

Walls of the Tympanic Cavity or Middle Ear

  • This cavity is shaped like a narrow six-sided box that has convex medial and lateral walls.
  • It has the shape of the biconcave lens in cross-section (like a red blood cell).

 

The Roof or Tegmental Wall

  • This is formed by a thin plate of bone, called the tegmen tympani (L. tegmen, roof).
  • It separates the tympanic cavity from the dura on the floor of middle cranial fossa.
  • The tegmen tympani also covers the aditus ad antrum.

 

The Floor or Jugular Wall

  • This wall is thicker than the roof.
  • It separates the tympanic cavity from the superior bulb of the internal jugular vein. The internal jugular vein and the internal carotid artery diverge at the floor of the tympanic cavity.
  • The tympanic nerve, a branch of the glossopharyngeal nerve (CN IX), passes through an aperture in the floor of the tympanic cavity and its branches form the tympanic plexus.

The Lateral or Membranous Wall

  • This is formed almost entirely by the tympanic membrane.
  • Superiorly it is formed by the lateral bony wall of the epitympanic recess.
  • The handle of the malleus is incorporated in the tympanic membrane, and its head extends into the epitympanic recess.

The Medial or Labyrinthine Wall

  • This separates the middle ear from the membranous labyrinth (semicircular ducts and cochlear duct) encased in the bony labyrinth.
  • The medial wall of the tympanic cavity exhibits several important features.
  • Centrally, opposite the tympanic membrane, there is a rounded promontory (L. eminence) formed by the first turn of the cochlea.
  • The tympanic plexus of nerves, lying on the promontory, is formed by fibres of the facial and glossopharyngeal nerves.
  • The medial wall of the tympanic cavity also has two small apertures or windows.
  • The fenestra vestibuli (oval window) is closed by the base of the stapes, which is bound to its margins by an annular ligament.
  • Through this window, vibrations of the stapes are transmitted to the perilymph window within the bony labyrinth of the inner ear.
  • The fenestra cochleae (round window) is inferior to the fenestra vestibuli.
  • This is closed by a second tympanic membrane.

 

The Posterior or Mastoid Wall

  • This wall has several openings in it.
  • In its superior part is the aditus ad antrum (mastoid antrum), which leads posteriorly from the epitympanic recess to the mastoid cells.
  • Inferiorly is a pinpoint aperture on the apex of a tiny, hollow projection of bone, called the pyramidal eminence (pyramid).
  • This eminence contains the stapedius muscle.
  • Its aperture transmits the tendon of the stapedius, which enters the tympanic cavity and inserts into the stapes.
  • Lateral to the pyramid, there is an aperture through which the chorda tympani nerve, a branch of the facial nerve (CN VII), enters the tympanic cavity.

The Anterior Wall or Carotid Wall

  • This wall is a narrow as the medial and lateral walls converge anteriorly.
  • There are two openings in the anterior wall.
  • The superior opening communicates with a canal occupied by the tensor tympani muscle.
  • Its tendon inserts into the handle of the malleus and keeps the tympanic membrane tense.
  • Inferiorly, the tympanic cavity communicates with the nasopharynx through the auditory tube.

Genioglossus Muscle

  • Origin: Mental spine of the mandible.
  • Insertion: Dorsum of the tongue and hyoid bone.
  • Nerve Supply: Hypoglossal nerve (CN XII).
  • Arterial Supply: Sublingual and submental arteries.
  • Action: Depresses and protrudes the tongue.

  • Long bones (e.g.. femur and humerus)
  • Short bones (e.g.. wrist and ankle bones)
  • Flat bones (e.g.. ribs)
  • Irregular bones (e.g.. vertebrae)

  • The palate has a rich blood supply from branches of the maxillary artery.

The Auditory Tube

  • This is a funnel-shaped tube connecting the nasopharynx to the tympanic cavity.
  • Its wide end is towards the nasopharynx, where it opens posterior to the inferior meatus of the nasal cavity.
  • The auditory tube is 3.5 to 4 cm long; its posterior 1/3 is bony and the other 2/3 is cartilaginous.
  • It bony part lies in a groove on the inferior aspect of the base of the skull, between the petrous part of the temporal bone and the greater wing of the sphenoid bone.
  • The function of the auditory tube is to equalise pressure of the middle ear with atmospheric pressure.

The Palate

  • The palate forms the arched roof of the mouth and the floor of the nasal cavities.
  • The palate consists of two regions: the anterior 2/3 or bony part, called the hard palate, and the mobile posterior 1/3 or fibromuscular part, known as the soft palate.

 

The Hard Palate

  • The anterior bony part of the palate is formed by the palatine process of the maxillae and the horizontal plates of the palatine bones.
  • Anteriorly and laterally, the hard palate is bounded by the alveolar processes and the gingivae.
  • Posteriorly, the hard palate is continuous with the soft palate.
  • The incisive foramen is the mouth of the incisive canal.
  • This foramen is located posterior to the maxillary central incisor teeth.
  • This foramen is the common opening for the right and left incisive canals.
  • The incisive canal and foramen transmit the nasopalatine nerve and the terminal branches of the sphenopalatine artery.
  • Medial to the third molar tooth, the greater palatine foramen pierces the lateral border of the bony palate.
  • The greater palatine vessels and nerve emerge from this foramen and run anteriorly into two grooves on the palate.
  • The lesser palatine foramen transmits the lesser palatine nerve and vessels.
  • This runs to the soft palate and adjacent structures.

The Nasopharynx

  • The nasal part of the pharynx has a respiratory function.
  • It lies superior to the soft palate and is a posterior extension of the nasal cavity.
  • The nose opens into the nasopharynx via to large posterior apertures called choanae.
  • The roof and posterior wall of the nasopharynx form a continuous surface that lies inferior to the body of the sphenoid bone and the basilar part of the occipital bone.
  • In the mucous membrane of the roof of the posterior wall of the nasopharynx is a collection of lymphoid tissue, known as the pharyngeal tonsil (commonly known as the adenoids).
  • The pharyngeal orifice of the auditory tube is on the lateral wall of the nasopharynx, 1 to 1.5 cm posterior to the inferior concha, and level with the superior border of the palate.
  • The orifice is directed inferiorly and has a hood-like tubal elevation over it called the torus of the auditory tube or the torus tubarius (L. torus, swelling).
  • Extending inferiorly from the torus is a vertical fold of mucous membrane, known as the salpingopharyngeal fold.
  • The collection of lymphoid tissue in the submucosa of the pharynx, posterior to the orifice of the auditory tube, is known as the tubal tonsil.
  • Posterior to the torus and the salpingopharyngeal fold, there is a slit-like lateral projection of the pharynx called the pharyngeal recess.
  • It extends laterally and posteriorly.

Explore by Exams