330 



ANATOMY AND PHYSIOLOGY 



covered with very sensitive skin on the outer surface, and mucous 

 membrane within (Fig. 210). 



The middle ear is the tympanum, or drum. It consists of a 

 small cavity in the petrous bone, on the inner side of the membrane 

 of the drum. Its height is barely half an inch, and the other 

 measurements are smaller still. It contains the little bones and 

 forms the beginning of the auditory tube. 



The auditory (or Eustachian) tube begins in the wall of the 

 middle ear and ends as a roll of cartilage opening into the pharynx. 



The tympanum is really an air chamber, since it communicates 



S emicircular 

 canals 



Drum membrane 



Cochlea 

 Cavity of tym- 

 panum or 

 drum 



\ 



Parotid gland 



Styloid process 



Internal 

 carotid artery 



Auditory tube 



FlG. 210.- 



-THE EAR. (Morris.) 



with the throat by the auditory (or Eustachian) tube, and both tube 

 and tympanum are lined with a continuation of the same mucous 

 membrane. An opening at the back of the tympanum leads into 

 the mastoid antrum, and through this, inflammation of the middle 

 ear frequently extends to the mastoid cavities. 



Note. The mucous membrane of the pharynx is continued 

 through the auditory tube into the tympanum, and through that 

 into the mastoid cells. Swelling of this membrane may occlude 



