LECT. XIX. PROPAGATION OF SOUND. 353 



bratory movement reaches the membrane of the tympanum, 

 which is stretched over the internal orifice of this tube. 

 Why, it may be asked, has the tympanum been added ? 

 Why is not the apparatus so arranged that the vestibule or 

 sac, in which the acoustic nerve is contained, should be in 

 contact with the membrane of the tympanum ? We have 

 no hesitation in replying that hearing could be effected 

 without this tympanum, as is the case in species of ani- 

 mals ; and, as has been said, with some men, in whom 

 this middle ear has been deficient, either from disease or 

 from natural conformation. It is obvious, however, on 

 physical grounds, that the middle part of the human ear 

 renders this organ more perfect and less exposed to un- 

 dergo alterations.. 



Let us, in the first place, speak of the manner in which 

 membranes vibrate. Savart has shown that, when they are 

 properly stretched, or put in proximity with an organ pipe, 

 or any stringed instrument producing a sound, they vi- 

 brate as if they were in contact with the sonorous bodies ; 

 and if they be covered with sand, we obtain the division 

 of these membranes into vibrating parts, separated by the 

 ordinary nodal lines. At each variation of sound, new 

 arrangements appear upon the membrane, and these di- 

 visions are more easily and quickly produced upon mem- 

 branes than upon plates of metal or glass. Savart has 

 likewise shown, that membranes alone present the pecu- 

 liarity of dividing in different ways under the influence of 

 the same sound ; all of them having the same form, di- 

 mension, and tension. Lastly, we may add, that by vary- 

 ing the degree of tension of a membrane, its manner of 

 dividing and vibrating for the same sound, is changed. 

 From all these facts, for which we are indebted to Savart, 

 we may infer that, in order to propagate sound in the inte- 

 rior of the ear, and m order to modify at pleasure the in- 

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