CHAP XVT1I.] OFFICE OF THE TYMPANUM. 91 



brane tense, it was much more difficult to produce manifest move- 

 ments in the grains of sand ; thus affording much reason to suppose 

 that the tensor tympani muscle is analogous in its use to the iris, 

 and destined to protect the organ from too strong impressions. 

 These experiments can be best tried on the membrana tympani of 

 the calf. 



In imitation of the mechanism by which the tension of the mem- 

 brana tympani is effected, and with a view to determine more de- 

 cisively the effects produced by variation of the tension of that mem- 

 brane, Savart constructed a conical tube (fig. 145), with its apex 

 truncated and covered by a layer of very thin Figf 145i 



paper, m, which was glued to the edge of the 

 opening. A little wooden lever, I, I, intro- 

 duced through an opening in the side of the 

 tube, and resting on the lower margin of this 

 opening, c, as a fulcrum, was used to vary the 

 tension of the membrane, one of its extremities 

 being applied to the under surface of the 

 membrane. By depressing the extremity of 

 the lever external to the tube, the inner one is raised, and thus the 

 membrane stretched to a greater or less degree, according to the 

 force used; on the other hand, by elevating the outer extremity, 

 the inner one is separated from the membrane, which is accordingly 

 restored to its original tension. This little lever is an imitation of 

 the handle of the malleus, which, under the influence of its muscles, 

 causes the variation in the tension of the membrana tympani. The 

 artificial tympanic membrane having been then covered with a layer 

 of sand, it was found that, under the influence of a vibrating glass, 

 used as in the former experiments, a manifest difference was pro- 

 duced in the movements of the grains of sand, by increasing the 

 tension of the paper ; the greater the tension, the less the height to 

 which the grains of sand were raised ; and these movements were 

 most extensive when the lever was withdrawn from contact, and 

 the membrane left to itself. 



From these experiments, Savart concludes that the membrana 

 tympani may be considered as a body thrown into vibration by the 

 air, and always executing vibrations equal in number to those of the 

 sonorous body which excites the oscillations of the air. But what 

 is the condition of the ossicles of the tympanum whilst the mem- 

 brane is thus in vibration ? The result of the following experiment 

 affords a clue to the answer to this question. To a membrane 

 stretched over a vessel, as in fig. 146, a piece of wood, a, b, uniform 



