528 THE SENSE OF HEARING. 



the posterior aperture of the nostrils. The cavity of the tym- 

 panum communicates posteriorly with air-cavities, the mastoid 

 cells in the mastoid process of the temporal bone ; but its only 

 opening to the external air is through the Eustachian tube 

 (c, Fig. 197). The walls of the tympanum are osseous, except 

 where apertures in them are closed with membrane, as at the 



FIG. 197. 



General view of the external, middle, and internal ear, as seen in a prepared sec- 

 tion through a, the auditory canal, b. The typanura or middle ear. c. Eustachian 

 tube, leading to the pharynx, d. Cochlea; and e. Semicircular canals and vestibule, 

 seen on their exterior, as brought into view by dissecting away the surrounding 

 petrous bone. The styloid process projects below, and the inner surface of the 

 carotid canal is seen above the Eustachian tubes (from Scarpa). 



fenestra rotunda, and fenestra ovalis, and at the outer part 

 where the bone is replaced by the membrana tympani. The 

 cavity of the tympanum is lined with mucous membrane, the 

 epithelium of which is ciliated and continuous with that of the 

 pharynx. It contains a chain of small bones (ossicula auditus), 

 which extends from the membrana tympani to the fenestra 

 ovalis. 



The membrana tympani is placed in a slanting direction at 

 the bottom of the external auditory canal, its plane being at 

 an angle of about 45 with the lower wall of the canal. It is 



