424 THE SENSORIAL FUNCTIONS. 



would expose the membrane of the ear-drum to 

 great inequalities of pressure at its outer and 

 inner surfaces, and endanger its being forced, 

 according to the state of the weather, either out- 

 wards or inwards, which would completely inter- 

 fere with the delicacy of its vibrations. Nature 

 lias guarded against these evils by establishing 

 a passage of communication between the tym- 

 panum and the external air, by means of a tube 

 (e), termed the JEustachian 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 

 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 ob- 

 structed, 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 admit- 

 tance to the internal ear ; and it is perliaps for 



* This opening is seen at e, in Fig. 382, p. 400, representing 

 a vertical and longitudinal section of the right nostril. 



