XVIII 



THE SPECIAL SENSES AND SENSATIONS 



261 



pressure the same on the inner and outer surfaces of 

 the drumhead. This tube and the entire middle ear 

 are lined with mucous membrane. Hence, it is easily 

 seen why a cold in the head, sore throat, catarrh, and 

 adenoid growths affect our hearing. Anything that 

 tends to close the Eustachian tubes or to obstruct the 

 air passages makes the drumhead less sensitive to sound 

 waves and impairs hearing. 



Close the mouth, pinch the nostrils firmly, and then swallow. 

 Because a little air is removed from the pharynx the eardrum is 

 pushed in, causing the peculiar cracking sound. 



179. The Inner Ear. There is no more delicate or 

 complicated organ in the body than the inner ear, which 

 is the real end organ 

 of hearing. It con- 

 sists of a series of 

 winding passages in 

 the temporal bone, 

 and may be divided 

 into three parts : the 

 small vestibule next 

 to the middle ear 

 with which the stapes 

 communicates ; the 

 three semicircular canals or winding passages connected 

 with the vestibule behind and above ; and the cochlea or 

 snail shell joined to the vestibule on the opposite side, 

 in front and below. All these parts are filled with a 

 liquid called perilymph, in which floats a saclike mem- 

 brane, the membranous labyrinth, that has about the 



FIG. 139. The inner ear, showing the shape of 

 the bony labyrinth (Zuppke). 



