222 THE SPECIAL SENSES. 



to force air through the latter. When this is done, a dis- 

 tinct crackle or clicking sound is perceived, due to the 

 movement of the membranes, and of the little bones of the 

 ear. 



99. The Eustachian tube serves, also, as an escape-pipe 

 for the fluids which form within the middle ear; and 

 hence, when its lining membrane becomes thickened, in 

 consequence of a cold, or sore throat, and the passage is 

 thus more or less choked up, the fluids are unable to escape 

 as usual, and therefore accumulate within the ear. When 

 this takes place, the vibrations of the membrane are inter- 

 fered with; the sounds heard appear muffled and indis- 

 tinct; and a temporary difficulty of hearing, which is 

 known as "throat-deafness," is the result. This result re- 

 sembles the effect produced by interrupting the vibrations 

 of a sonorous body, such as all are familiar with ; if the 

 finger be placed upon a piano-string or bell when it is 

 struck, the proper -sound is no longer fully and clearly 

 emitted. But the primary use of this tube is to afford a 

 free communication between the middle ear and the exter- 

 nal atmosphere, and thus secure an equal pressure upon 

 both sides of the membrane of the drum of the ear, however 

 the density of the atmosphere may vary. If, from undue 

 tension of the membranes, pain is experienced in the ears, 

 when ascending into a rare atmosphere, as in a balloon, or 

 descending into a dense one, as in a diving-bell, it may be 

 relieved by repeating the act of swallowing, from time to 

 time, in order that the inner and outer pressure may thus 

 be promptly equalized. 



100. The Internal Ear, or Labyrinth. The most 

 essential part of the organ of hearing is the distribution of 

 the auditory nerve. This is found within the cavity of the 

 internal ear, which, from its exceedingly tortuous shape, 



99. What other purpose does the Eustachian tube serve * How is this shown ? 

 " Throat-deafness* *" Primary use of the Eustachian tube ? 



100. The essential part of the organ of hearing ? Its location ? Formation ? 



