426 PRACTICAL ANATOMY. 



tympaui secondaria). The promontory, in front of the 

 fenestra rotunda, is a hollow, dome-like elevation, formed 

 by the first turn of the cochlea ; it presents two or three 

 grooves, for the tympanic plexus. 



The pyramid, cone-shaped, is placed behind the 

 fenestra rotunda. Tt is hollow, and contains the stapedius 

 muscle. At the apex of the pyramid is a minute 

 orifice, which transmits the tendon of the stapedius. The 

 aquaBductus Fallopii skirts the upper and posterior por- 

 tion of the inner wall. It contains the facial nerve. 

 The posterior wall presents the openings of the mastoid 

 cells. The anterior wall is separated from the carotid 

 canal by a thin plate of bone. It presents the opening 

 of the Eustachian tube, the processus cochleariformis, 

 and the opening of the canal for the tensor tympani 

 muscle. 



The Eustachian tube is about one and one-half inches 

 long, and is directed downward, forward, and inward. 

 It is an osseo-cartilaginous canal ; the osseous portion is 

 about one-half inch in length ; the cartilaginous portion, 

 one to one and one-half inches long, opens trumpet-like 

 into the pharynx. The processus cochleariformis is a 

 thin plate of bone, which separates the Eustachian tube 

 below from the canal above and transmits the tensor 

 tympani muscle. 



The membrana tympani is an oval, translucent mem- 

 brane, which is placed obliquely at the bottom of the 

 external auditory canal. It presents three layers, ex- 

 ternal, or cuticular; middle, or fibrous; and internal, or 

 mucous. The handle of the malleus descends between 

 the fibrous and mucous layers to about the centre. 

 Externally it is concave, but it changes its position 

 slightly under the pressure of the air within the tym- 

 panum. 



