VAN 121: Veterinary Gross Anatomy-II (2+2)

MIDDLE EAR

• The middle ear comprises the tympanic cavity or tympanum with its contents and the eustachian tubes.
• The tympanum is the cavity excavated in the bulla tympanica of the petrous temporal bone. It is situated between the tympanic membrane and internal ear.
  • It is a laterally com- pressed air cavity lined by mucous membrane and communicates with the pharynx by the eustachian or pharyngo-tympanic tube.
  • The cavity presents a main part the atrium, which lies medial to the tympanic membrane and a recessus epitympanicus, which lies above the level of the tympanic membrane, containing the upper part of the malleus and a greater part of the incus and a larger recess below the bulla tympanica.
  • The cavity has six walls and the atrium communicates ventrally with the air cells of bulla tympanica.
  • The roof or tegmental wall is crossed in its medial part by the facial nerve as the facial canal is deficient below and the nerve is covered by mucous membrane.
  • The floor is concave and thin.
  • It is crossed by delicate curved ridges that radiate from the annulus tympanicus.
  • The anterior or tubal wall is narrow and is pierced by the slit-like opening of the eustachian tube. The posterior wall presents no special features.
  • The lateral wall is formed by the membrana tympani, which closes the medial end of the external acoustic meatus and thus forms a septum between the external and middle ear. It is an oval disc that slopes downwards and inwards.
  • The circumference of this is attached in a groove -the sulcus tympanicus in a thin ring of bone the annulus tympanicus that completely surrounds it.
  • The handle of the malleus is attached to the internal face of the membrane and draws the central part inward, producing a slight concavity of the outer surface.
  • The periphery of the membrane is thickened by fibrous tissue, annulus fibrosus.

• The tympanic membrane or ear drum is semitransparent and consists of three layers
  • Outer thin cutaneous layer
    • Outer thin cutaneous layer, a prolongation of the lining of external acoustic meatus
  • Middle fibrous layer
    • Middle fibrous layer the membrana propria consisting of two sets of fibres, external radiating from the handle of malleus and internal circular
  • Internal mucous layer
    • Internal mucous layer, a part of the mucous membrane lining the tympanic cavity. The medial or labyrinthine wall separates it from the internal ear.   
• The promontory is a distinct eminence near the centre, which corresponds to the first basal coil of cochlea, and is marked by a faint groove for superficial petrosal nerve.
•  Above and behind the promontory it presents a reniform opening the fenestra vestibule (oval window), which is closed by the footplate of stapes.
• The fenestra cochleae (round window) an irregularly oval opening is below and behind it and is closed by the secondary tympanic membrane which separates the tympanum from the scala tympani of the cochlea.
• The facial nerve runs along the medial wall in a canal immediately above the fenestra vestibuli. This canal is immediately above the fenestra vestibuli and causes a prominence.
• The auditory ossicles form a chain of bones extending from the lateral to the medial wall of the cavity.

• These from without inward are  Malleus,  Incus and Stapes.
  • The malleus (hammer) has a head, neck, handle (manubrium) and two processes. The head lies in the attic or recessus epitympanicus and presents posteromedially a facet for the incus. The handle is attached to the membrana tympani.
  • The incus (anvil) has a body and two processes. The body articulates with the malleus while the long process has attached to it the os lenticulare, which is a small nodule of bone articulating with the stapes.
  • The stapes (stirrup) has a head, two crura and a base. The head meets os lenticulare and the base is attached to the fenestra vestibuli.
• Muscles: The tensor tympani and stapedius muscles consist of skeletal muscle fibres.
• The tensor tympani lies for the most part in a canal just above the auditory tube in the anterior wall of the tympanic cavity.
• It ends in a tendon, which bends sharply around the end of a bony canal and passes laterally across the tympanic cavity to be inserted to the manubrium inward and thus makes the tympanic membrane tense.
• The stapedius muscle lies within a small conical bony projection the pyramidal eminence on the posterior wall of the tympanic cavity.
• Its tendon passes through a minute aperture in the summit of the eminence and is inserted into the posterior surface of the neck of stapes.
• Besides the muscles, there are several ligaments.
• The chorda tympani nerve passes across the tympanic cavity its entire lateral wall directly over the manubrium of the malleus.
• Lining the tympanic cavity and investing all the structures contained within, is a membrane, the tympanic mucosa.
• This consists of a thin connective tissue propria covered by a simple squamous epithelium in the ventral portion of the tympanic cavity and on the tympanic membrane the auditory ossicles and the secondary tympanic membrane.
• The rest of the cavity is lined by ciliated columnar epithelium.
• The Eustachian (pharyngotympanic) tube extends from the tympanum to the pharynx.
• It is about 2 inches long. Its upper end is at the medial aspect of the root of the styloid process of petrous temporal and it communicates with the anterior part of the tympanic cavity by a small slit like opening. It consists of bony and cartilaginous parts.
• The tube is complete for about an inch and its lumen is a capillary space, this is the bony part.
• Further downwards it is a groove and is completed below by cartilage.
• Pharyngeal opening is small and is on the superoposterior part of pharynx close to the base of cranium.
• The mucosa bears pseudostratified ciliated epithelium containing goblet cells.