294 



PHYSIOLOGY FOR DENTAL STI DKNTS. 



half times stronger. The increase in force with which the 

 movements of the tympanic membrane arc conveyed to the oval 

 window is still further magnified by the fact that the latter is 

 only one-twentieth the size of the former. It is by these move- 

 ments at the oval window that waves are set up in the fluid 

 occupying the uppermost membraneous tube of the cochlea and 

 thus acting on the basilar membrane. The tympanic cavity or 



Fig. 58. Tympanum of right side with the auditory ossicles in place (.Mor- 

 ris) : 1, incus (like bicuspid tooth) with one process (,,1) attached to wall of 

 tympanum and the other running downwards to articulate at 9 and 8, the 

 stapes ; 10, head of malleus attached to tympanic membrane. ( From How- 

 ell's Physiology.) 



tympanum across which the chain of ossicles stretches is kept 

 at atmospheric pressure by the Euxldcliian tub< , which connects 

 it with the posterior nares. 



Deafness may be din to tin foll<>irin</ causes: 



1. Rupture of the tympanic membrane. 



2. Ankylosis or stiffening of the joints between the ossicles 



