HEARING. 



301 



these evils by establishing a passage of communication be- 

 tween the tympanum and the external air, by means of a 

 tube (E,) termed the Eustachian tube, which begins by a 

 small orifice from the inner side of the cavity of the tympa- 

 num, and opens by a wide mouth at the back of the nos- 

 trils.* This tube performs the same office in the ear, as the 

 hole which it is found necessary to make in the side of a 

 drum, for the purpose of opening a communication with 

 the external air; a communication which is as necessary for 

 the functions of the ear, as it is for the proper sounding of 

 the drum. We find accordingly that a degree of deafness is 

 induced whenever the Eustachian tube is obstructed, which 

 may happen either from the swelling of the membrane 

 lining it, during a cold, or from the accumulation of secre- 

 tion in the passage. It is also occasionally useful as a chan- 

 nel through which sounds may gain admittance to the inter- 

 nal ear; and it is perhaps for this reason that we instinct- 

 ively open the mouth when we are intent on hearing a very- 

 faint or distant sound. 



On the side of the cavity of the tympanum, which is op- 

 posite to the opening of the Eustachian tube, is situated the 

 beginning of another passage, leading into numerous cells, 

 contained in the mastoid process of the temporal bone, and 

 therefore termed the mastoid cells: these cells are likewise 



391 



393 



filled with air. The innermost side of the same cavity, that 

 is, the side opposite to the ear-drum, and which is shown in 



* This opening is seen at n, in Fig. 9JK, p. ? representing a vertical and 

 longitudinal section of the right nostril. . 



