THE EAR 



363 



and jarring it. In the skin which lines this canal are numerous 

 glands that secrete a substance which partially evaporates 

 and leaves what is called ear wax. This seems to have little 

 function, but it should never be interfered with except by a 

 competent physician. 



The Middle Ear. The middle ear is a space just inside the 

 ear drum (Fig. 181) and is scientifically called the tym- 

 panum. The cavity 

 is not large and is 

 surrounded by bone 

 on all sides save at 

 a few points, which 

 will be especially 

 noticed later. On the 

 lower side of this cav- 

 ity is an opening lead- 

 ing into the Eusta- 

 chian tube, which 

 passes down to the 

 throat. On the inner 

 side of the tympanum 

 are two small open- 

 ings, which lead to the 

 inner "ear, but which 

 are closed by mem- 

 branes; the upper one is the foramen ovale, the lower, 

 the foramen rotundum. At the upper part of the tympanum 

 are some minute pores leading into spaces, called the mastoid 

 cavities, in the surrounding bone. Reaching across the tym- 

 panic cavity is a chain of three little bones; Fig. 181. 



The Eustachian Tube. The Eustachian tube serves two 

 very important functions. First, it allows air to go in and 

 out, between the ear and the pharynx. At first thought it 

 seems odd that there should be any connection between the 

 ear and the throat, but such connection is necessary to keep 



FIG. 181. DIAGRAM 



Showing the structure of the ear. In the lower figure 

 the inner ear is shown natural size. In the upper 

 figure the external ear is shown much too small 

 relatively to the size of the internal structures. 

 The oblique shading represents bone. A, external 

 meatus; B, utricle; C, saccule; D, semi-circular 

 canal; E, nerve; F, cochlea; G, Eustachian tube; 

 H, tympanic membrane; /, cochlea. 



