////: MIIU'l.i: I'M! 



7 T.V/M \/ .U. 



841 



tlmt liniiiL' tin- jiluirynx l>y means of a cartilaginous canal -the Euttachian tube, 

 which conveys the external air to the middle ear. 



We will glance briefly at the anatomical characters of the parts enu- 

 merated. and which enter into the formation of the middle ear. 



1. Membrana 



Situated on the external wall of the middle enr, which it separates from 

 the bottom of the auditory canal, this membrane is oval in shape. It is 

 thin, dry, and capable of vibrating. Its inner face, inclining inwards and 

 slightly convex, is adherent to the handle of the malleus. Its external 

 is slightly concave (towards the meatus). The circumference is fixed 

 in a bony frame named the tympanal circle, which is sharply defined, but 



Fig. 394. 



RIGHT TVMrvM" < AVITV OK TIIK. H'H:-. > I \l: : ANTERIOR PI.AXK, VKU I 



\si> 1 1: \-<-\ can M 1 1\. 



A, Auditory c.in-il : u, Mrmbrann t vin|>:nii : e. M:illiMi--: i>. lut-n^: i:. Os orbirtilnre ; 

 :<iiil cell--: i ..v.ilis; i. Vcstit.uli-: .1. K. 1., Outlim- i.f 



the semicircul-ir ennuis; u, ('o<-hlen; x, Commencement <<i tlie tyinpaiiii- scala. 



incomplete at its upper part, and enveloped by the mastoid culls, whose 

 cavities radiate around this circle. 



Although very thin, this membrane is composed <>f three layers: a 



