HEARING. 379 



expose the membrane of the ear-drum to great 

 inequalities of pressure on its outer and inner 

 surfaces, and endanger its being forced, according 

 to the state of the weather, either outwards or 

 inwards ; conditions which would affect its degree 

 of tension, and alter the character of its vibrations. 

 Nature has guarded against these evils by esta- 

 blishing a passage of communication between 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 tympanum, and opens by a wide mouth at 

 the back of the nostrils.* This tube performs in 

 the ear an office similar to that of 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, accord- 

 ingly, that a degree of deafness is induced when- 

 ever 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 secretion in the passage. It is also occasionally 

 useful as a channel through which sounds may gain 

 admittance to the internal ear ; and it is perhaps 

 for this reason that we instinctively 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 opposite to the opening of the Eustachian 

 tube, is situated the beginning of another passage, 



* This opening is seen at k, in Fig. 38'2, p. 357, representing a 

 veitici.ll and longitudinal section of the right nostril. 



