AXTERIOR ASPECT OF HEAD AND NECK. 



THE MIDDLE EAR. 



Tympanic Cavity. This is a small chamber in the tem- 

 poral bone, between the external auditory meatus and the 

 internal ear. It consists of the tympanum proper (atrium), 

 lying next the ear drum; and of the recessus epitympanicus, 

 lying above the ear drum. Its roof is a thin bony plate (teg- 

 men tympani), which separates it from the cranial cavity and 

 is prolonged over the mastoid antrum and canal for the Tensor 

 tympani, as their roof. Its floor is a narrower and thin bony 

 plate which separates it from the jugular fossa. The orifice for 

 Jacobson's nerve lies near the junction of the floor and inner 

 wall. [748] 



The posterior wall presents, from above downward, the fol- 

 lowing structures: The opening from the epitympanic recess 

 into the mastoid antrum. A depression (fossa incudis), for 

 the short process of the incus, at the lower and back part of 

 the epitympanic recess. A conical process (pyramid), which is 

 pierced at its summit by the canal for the Stapedius; this 

 canal descends in front of the aqueductus Fallopii and opens 

 in front of the stylo-mastoid foramen. The apertura tympanica 

 canaliculi chordae, a small orifice just internal to the upper 

 part of the posterior edge of the drum. The prominentia 

 styloidea, an inconstant rounded eminence just below the 

 latter, and produced by the styloid process. [749] 



The anterior wall is narrow and presents two canals, the 

 upper for the Tensor tympani, and the lower forming the bony 

 part of the Eustachian tube; a bony lamella (processus coch- 

 leariformis) separates them. The outer wall is chiefly formed 

 by the membrana tympani, which is attached in a groove (sul- 

 cus tympanicus) contained in a bony ring; the ring is defi- 

 cient above (notch of Rivinus). In front of this ring, supe- 

 riorly, is the Glaserian fissure ; near the inner end of this fissure 

 is the canal of Huguier (iter chordae anterius) . The membrana 

 tympani is elliptical, thickened at its edges, and formed of 

 7 [97] 



