DISSECTING MANUAL. 



fibrous tissue covered by skin and mucous membrane. It is 

 concave externally, with the apex (umbo) of the concavity 

 over the tip of the handle of the malleus. Two prolongations 

 (anterior and posterior malleolar folds) of the membrane pass, 

 at the extremities of the notch of Rivinus, to the short process 

 of the malleus. The triangular part of the membrane above 

 these folds is lax (pars flaccida), but the rest is tense (pars 

 tensa). [750] 



The inner wall presents the following structures: An emi- 

 nence (promontory) over the cochlea is grooved for the tym- 

 panic plexus. Above and behind this a funnel-shaped recess 

 (fossula fenestra; vestibuli) leads into the vestibule through 

 an oval opening (fenestra ovalis), which is occupied by the foot 

 of the stapes. Above this is a prolongation of the processus 

 cochleariformis, and also a ridge (prominentia canalis facialis) 

 produced by the Fallopian aqueduct. Below and behind the 

 promontory is a funnel-shaped recess (fossula fenestrae coch- 

 leae) , which leads into the cochlea through an opening (fenestra 

 rotunda) closed by the membrana tympani secundaria. Be- 

 tween the fenestra ovalis and fossula rotunda is a small cir- 

 cular depression (sinus tympani) lying over the ampulla of the 

 posterior semicircular canal. [752] 



Mastoid Antrum and Air Cells. The antrum is a cavity 

 in the temporal bone lying behind the tympanum and roofed 

 in by the tegmen tympani. The air cells vary in number and 

 size ; they open into the antrum and are found not only below 

 and behind it, in the mastoid process, but also internal to and 

 above it. [752] 



Eustachian Tube. This canal runs downward and inward 

 from the tympanum to the upper part of the naso- pharynx. It 

 is formed by bone (pars ossea) externally and by fibre-cartilage 

 (pars cartilaginea) antero-internally, being narrowest (isth- 

 mus) at their junction. The pars ossea is half an inch long and 

 lies in the petro-squamous angle of the temporal. The pars 

 cartilaginea is about an inch long and inclines a little more 



[98] 



