THE EAR 203 



downwards and forwards over the antero-inferior quadrant of 

 the tympanic membrane (Fig. 78). 



Posterior to the handle of the malleus the shadow of the 

 long process of the incus may be made out. It is parallel to 

 the former but lies on a slightly deeper plane, and is not in 

 direct contact with the membrane. As a result, it can only 

 be observed under favourable conditions. 



The " cone of light " is taken as a guide when the operation 

 of paracentesis is carried out for the evacuation of pus from 

 the middle ear. The incision is made immediately posterior to, 

 and on a level with, the "cone of light," so that it passes 



Crus longum 

 of incus 



Malleus 



Site for para- 

 centesis tympani 



FIG. 78. Lateral Aspect of Right Tympanic Membrane. 

 NOTE. The "cone of light" occupies the antero-inferior quadrant of the membrane. 



through the postero-inferior quadrant. This area is chosen, as 

 it is well removed from the ossicles and the chorda tympani. 

 Further, on account of the obliquity of the membrane, good 

 drainage is afforded by an opening in this position. 



The medial wall of the middle ear is formed by a part of 

 the petrous portion of the temporal bone, and it separates the 

 cavity from the internal ear. Nearly the whole extent of this 

 wall is occupied by a well-marked elevation, termed the 

 promontory, which is produced by the first coil of the cochlea. 

 At the postero-superior corner of the promontory, the foot- 

 piece of the stapes fills in an oval aperture in the bone and it 

 is in contact with the perilymph of the internal ear. At the 



