212 



THE HEAD AND NECK 



the membrane is selected because it is furthest removed from the 

 handle of the malleus and the chorda tympani, and because it 

 affords good drainage. The nerve runs downwards and forwards 

 across the deep surface of the membrane, lateral to the crus 

 longum of the incus, but medial to the neck of the malleus. 

 The point of the knife must not be pushed too deeply through 

 the membrane as the tympanum is only one-eighth of an inch 

 wide at this point, and the fenestra of the cochlea (fenestra 

 rotunda), which lies on its medial wall, may be injured. 



The Tympanum is about half an inch in length, and varies 

 from a tenth to a sixth of an inch in width. Its uppermost part, 

 known as the Epitympanic Recess (Attic), lies above the level of 



Membrana flaccida 



Anterior malleolar 



plica 



Manubrium mallei 



Antero-superior 

 quadrant 



Antero-inferior 

 quadrant 



Posterior malleolar 



plica 



Lateral process of 



malleus 



Long crus of incus 



Postero-superior 

 quadrant 



Postero- inferior 

 quadrant 



Cone of light 



FiG. 64. Left Tympanic Membrane (as viewed from the external 

 acoustic meatus). x 3. 



the tympanic membrane, and contains the head of the malleus 

 and the body of the incus. 



The Anterior Wall of the tympanum is formed by a thin 

 plate of bone, which separates the tympanic cavity from the 

 carotid canal. In the lateral part of this wall is situated the 

 opening of the auditory tube, which leads downwards, forwards, 

 and medially to the nasal pharynx. In the child the direction 

 of the tube is practically horizontal, the downward inclination 

 being very slight. The Koof is formed by the legmen tympani, 

 which separates the tympanum, the auditory tube, and the 

 tympanic (mastoid) antrum from the middle cranial fossa and 

 the temporal lobe of the brain. Its lateral border corresponds 

 to the line of the petro-squamous suture, which may remain 

 unossified until the end of the second year. Up to that age 

 inflammatory conditions of the brain or meninges may be caused 



