THE EAR AND THE SENSE OF HEARING 127 



the tube of the meatus are placed the wax glands with 

 their ducts. 



169. The Middle Ear, or Tympanum, is an irregular cavity 

 in the temporal bone, lined with mucous membrane which 

 is supplied with small glands. By means of the Eusta- 

 chian tube, which passes from the lower back part to the 

 pharynx, or upper cavity of the throat, the tympanum 

 communicates with the external air, and so equalizes the 

 pressure upon the two sides of the membrane of the tym- 

 panum which separates the middle from the external ear. 



Opposite the membrane of the tympanum are two 

 smaller openings into the chamber of the inner ear, called 

 the round window (fenestra 

 rotunda^), and the oval win- 

 dow (fenestra ovalis^). These 

 are closed, however, by thin 

 membranes. Three small 

 bones, called the auditory 

 ossicles, form a chain across 

 the middle ear (Figs. 74 and 

 75). The malleus, or hammer, 

 is attached by one end to the 



membrane of the tvmpanum. 



J ' Fig. 75. The ossicles of the ear. 



while the other end articu- 

 lates with the incus, or anvil bone. The incus articulates 

 with the third bone, the stapes, or stirrup bone, at the top 

 of its arch. The foot plate of the stapes fits into the open- 

 ing on the inner side of the tympanum called the oval 

 window, and is attached to the membrane which closes it. 



170. The Internal Ear, or Labyrinth, is the essential part 

 of the organ of hearing, the others being merely con- 

 ductors of sound waves. The labyrinth is an irregular 

 chamber in the rocky part of the temporal bone (Figs. 74 



