HEARING. 



573 



Fig. 262. 



acted and rendered the membrane tense, it was 

 much more difficult to produce manifest move- 

 ments in the grains of sand ; thus affording 

 much reason to suppose the tensor tympani 

 muscle is analogous in its use to the iris, and 

 destined to preserve the organ from too strong 

 impressions. This experiment can be best tried 

 on the membrana tympani of the calf. 



In imitation of the mechanism by which the 

 tension of the membrana tympani is effected, 

 and with a view to determine more decisively 

 the effects produced by variation of the tension 

 of that membrane, Savart constructed a conical 

 tube (Jig. 262), with its 

 apex truncated and co- 

 vered by a layer of very 

 thin paper (?/?), which 

 was glued to the edge of 

 the opening. A little 

 wooden lever (/ /), intro- 

 duced through an open- 

 ing 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. It is evident 

 that, by depressing the extremity of the lever 

 that was external to the tube, the inner one 

 would be raised, and thus the membrane 

 stretched to a greater or less degree according 

 to the force used ; on the other hand, by ele- 

 vating the outer extremity, the inner one was 

 separated from the membrane, which was ac- 

 cordingly restored to its original tension. This 

 little lever was employed in imitation of the 

 handle of the malleus, which under the in- 

 fluence of its muscles causes the variation in 

 the tension of the membrana tympani. The 

 artificial tympanic membrane then having been 

 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 dif- 

 ference was produced 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 when the lever was withdrawn from contact, 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 num- 

 ber to those of the sonorous body which gave 

 rise to the oscillations of the air. But what is 

 the condition of the ossicles of the tympanum 

 whilst the membrane is thus in vibration? The 

 result of the following experiment affords a 

 clue to the answer of this question. To a 

 membrane stretched over a vessel, as in fig. 

 263, a piece of wood (a 6) uniform in thick- 

 ness is attached, so that the adherent part shall 

 extend from the circumference to the centre of 

 the membrane, while the free portion may 

 project beyond the circumference. When 

 a vibrating glass is brought near this mem- 

 brane, very regular figures are produced, which 



Fig. 263. 



however are modified by the presence of the 

 piece of wood, and the vibrations of the mem- 

 brane are communicated to the piece of wood, 

 on which likewise regular figures may be pro- 

 duced. The more extensive the membrane, the 

 longer and thicker may be the piece of wood 

 in which it can excite oscillations, and Savart 

 states that, with membranes of a considerable 

 diameter, he has produced regular vibrations 

 in rods of glass of large dimensions. The 

 oscillations of the piece of wood are much 

 more distinct when the adherent portion is 

 thinned down, as in c d, jig. 264, by which it 



Fig. 264. 



seems, as it were, more completely identified 

 with the membrane, and consequently the 

 oscillations of this latter are communicated di- 

 rectly to the thinned portion of the wood, and 

 thence extended to the thick portion a : sand 

 spread upon a will exhibit active movements, 

 and will produce very distinct nodal lines. Thus 

 it may be inferred that the malleus participates 

 in the oscillations of the tympanic membrane ; 

 and these vibrations are propagated to the incus 

 and stapes, and thus to the membrane of the 

 fenestra ovalis. The chain of ossicles then evi- 

 dently performs the office of a conductor of 

 oscillations from the membrana tympani to the 

 membrane of the fenestra ovalis ; but the mal- 

 leus likewise has the important function under 

 the influence of its muscles of regulating the 

 tension of the tympanic membrane ; and to 

 allow of the changes in the position of this 

 bone necessary for that purpose, we find it 

 articulated with the incus by a distinct di- 

 arthrodial joint, and between this latter bone 

 again and the stapes there exists another and 

 a similar joint. This mobility then of the 

 chain of bones, and the muscular apparatus 

 of the malleus and stapes are totally irrespec- 

 tive of the conducting office of the bones, but 

 have reference to the regulation of the tension 

 of the membrane of the tympanum as well as 

 of that of the fenestra ovalis.* 



We have already seen how the muscle of 



* The experiments of Savart above detailed have 

 been several times carefully repeated by me with 

 results precisely similar. 



