354 HEARING. LECT. XIX. 



tensity of the vibratory movements, it is useful to close the 

 auditory passage with a tense membrane, and to add to the 

 interior of the organ an apparatus adapted for propagating 

 vibrations to parts in contact with the acoustic nerve, and 

 which, at the same time, can at will vary the tension of the 

 membrane of the tympanum. 



Here is a common ear-trumpet, on the aperture of which 

 is fixed a membrane ; upon this membrane a small wooden 

 rod is glued, as in the experiments of Savart and Muller. 

 By means of this, the tension of the membrane can be 

 easily altered. If I listen to a sound by applying the trum- 

 pet to my ear, I perceive a great difference in its intensity, 

 according as the membrane is rendered more or less tense : 

 when it is very much so, the sound is remarkably weaker 

 than that obtained by relaxing the membrane. In the first 

 case, perhaps, the membrane is more easily divided into 

 vibrating segments, which produce sounds in unison with 

 the primitive note, but not so strong. In the human ear 

 we can vary the tension of the membrane in two different 

 ways : either by the chain of ossicles (the effect produced 

 by the wooden stick in our preceding experiment;) or by 

 modifying the density and elasticity of the air contained in 

 the tympanum. This latter method, which is not the natu- 

 ral one, could only be effected by a certain degree of dex- 

 terity, and by making a violent effort. It is important, 

 then, to have in the middle ear a free and constant com- 

 munication between it and the exterior air : this is the office 

 of the Eustachian tube, which opens at one extremity into 

 the tympanum, and at the other into the fauces. In this 

 way the air of the tympanum has a constant degree of 

 moisture, while its elasticity is the same as that of atmos- 

 pheric air. The physical properties of the membrane of 

 the tympanum, as well as of the membranes of the fenes- 

 trse, rotunda, and ovalis, are thus preserved. 



