256 



THE HUMAN MECHANISM 



19. The middle ear ; the Eustachian tube. The tympanic 

 membrane separates the outer from the middle ear, or tym- 

 panum, a small cavity lying in the temporal bone of the 

 skull and communicating with the throat or pharynx by 

 means of the Eustachian tube. The air which it contains 

 is consequently under the same pressure as that of the 



FIG. 100. Diagram of the ear 



A, the auditory canal, leading to the tympanic membrane B; C, cavity of the 

 tympanum, communicating by the Eustachian tube with the pharynx D \ E, semi- 

 circular canals ; F, cochlea ; G, auditory nerve 



atmosphere without, and the tympanic membrane is not 

 normally bulged inward or outward by inequality of pres- 

 sure on its two sides. The opening of the Eustachian tube 

 into the pharynx is, however, closed except when one swal- 

 lows, and hence swallowing often relieves the drum from 

 undue pressure of air in the middle ear. 



The cavity of the tympanum also communicates with a 

 network of spaces, or sinuses, in the temporal bone. Because 



