THE MIDDLE EAR OR TYMPANUM. 



841 



that lining the pharynx by means of a cartilaginous canal the Eustachian 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 Tympani. 



Situated on the external wall of the middle ear, 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 

 face 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 TYMPANIC CAVITY OF THE HORSE ? S EAR ; ANTERIOR PLANE, VERTICAL 

 AND TRANSVERSE SECTION. 



A, Auditory canal ; B, Membrana tympani ; c, Malleus ; D, Incus ; E, Os orbiculare ; 

 F, Stapes ; G, Mastoid cells ; H, Fenestra ovalis ; i, Vestibule ; J, K, L, Outline of 

 the semicircular canals ; M, Cochlea ; N, Commencement of the tympanic scala. 



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

 cavities radiate around this circle. 



Although very thin, this membrane is composed of three layers: a 



