Scutum is present in middle ear –
Correct Answer: Lateral wall
Description: Ans. is 'b' i.e., Lateral wallBondaries of middle ear cavityo The middle ear cavity may be described as a six-sided box with a roof, a floor, a medial wall, a lateral wall, an anterior wall and a posterior wall.Roof or Tegmental wall (Paries tegmentalis)o It is formed by a thin plate of bone called the tegmen tympani It separates the tympanic cavity from middle cranial fossa. It is situated on the anterior surface of the petrous portion of temporal bone.Floor or Jugular wall (Paries jugularis)o It consists of a thin plate of bone called the fundus tympani. It separates the tympanic cavity from the jugular bulb. It presents, near medial (labyrinthie) wall, a small aperture for the passage of the tympanic branch of the glossopharyngeal nerve.Anterior or Carotid wall (paries carotica)o It is formed by thin plate of bone which separates the tympanic cavity from internal carotid artery. Through the opening of carotid canal (Glasserian fissure) passes superior and inferior caroticotympanic nerves and tympanic branch of internal carotid artery. Middle third has opening for Eustachian tube. Just above it is a canal containing tensor tympani muscle which extends to the medial wall to form a pulley called as processus cochleariformis.Posterior wallo It lies close to the mastoid air cells and has follow ing featuresA bony projection called the pyramid which contains stapedius muscle.There is an opening for exit of chorda tympani nerve.Aditus is an opening through which attic communicates with the mastoid antrum.Facial nerve runs in the posterior wall just behind the pyramid.Facial recess, also known as posterior sinus or suprapyramidaf recess is a depression in the posterior wall lateral to the pyramid. It is bounded medially by the vertical part of VII nerve, laterally by chorda tympani and above by the fossa incudis. Surgically, facial recess is important as direct access can be made through this into the middle ear without disturbing posterior canal wall (in intact canal wall technique).Lateral or outer or membranous w all (paries membranacea)o It is mainly formed by tympanic membrane and also partly by the ring of bone into which this membrane is inserted, i.e. scutum. The ring of bone is incomplete at its upper part, forming a notch (notch of Rivinus).Medial or inner or labyrinthie w all (paries labvrinthica)o It is formed by labyrinth and separates the middle ear cavity from internal ear. It has following featuresA bulge called as promontory formed by basal turn of cochlea.Fenestra vestibuli (oval window) lies posterosuperior (behind and above) to the promonton.- and opens into scala vestibuli. It is occupied by foot plate of stapes fixed by annular ligament. Its, size on average is 3 -25 mm long & 1 -75 mm wide.Fenestra cochleae (round window) lies posteroinferior to the promontory and opens into scala tympani of cochlea. It is closed by secondary tympanic membrane. The round window is closest to ampulla of posterior semicircular canal. Round window is a triangular opening. Its diameter is between PS to 2-3 mm.The round window opening is separated from oval window opening by a posterior extension of the promontory know n as subiculum.Occasionally another ridge of bone, the ponticulus, leaves the promontory above the subiculum and runs to the pyramid on the posterior wall of the cavity.Prominence of facial nerve canal (known as fallopian canal) lies above the fenestra vestibuli curving downwards into posterior wall of middle ear.Anterior to oval window lies a hook-like projection called the processus cochleariformis for tendon of tensor tympani. The cochleariform process marks the level of the Genu of the facial nerve which is an important landmark for surgery of the facial nerve.Medial to the pyramid is a deep recess called sinus tympani which is bounded by the subiculum below and the ponticulus above. It is the most inaccessible site in the middle ear & mastoid. Its importance is that cholesteatoma which has extended upto it is difficult to eradicate.
Category:
Anatomy
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