THE SENSE OF HEAKING 153 



size and tension of the membrane. But witli alternate waves of conden^^ation 

 and rarefaction, the tympanum vibrates only under their influence, and as these 

 vibrations will be exactly synchronous with those of the string, which, in our 

 previous example, we supposed to be the source of sound, their number in a 

 second of time will, in like manner, be four hundred inwards and four hundred 

 outwards. The form of the vibrations remains the same ; whether the magni- 

 tude of the displacements of the particles of air in contact with the tympanum 

 be great or small, nothing changes with that magnitude but the depth of the re- 

 sulting curvatures. If the air-wave does not strike the whole surfiice of the 

 tympanum perpendicularly and at the same time, but impinges obliquely by 

 striking first a part near the edgCi a convex curvature ensues, but its form will 

 be somewhat different. Though this be commonly the case on account of the 

 oblique position of the tympanum to the auditory canal, we will not enter into 

 a closer analysis of the different forms of curvature which would result from this 

 circumstance, since nothing is thereby essentially changed, and the following 

 explanations will be rendered more generally intelligible by being made in ref- 

 erence only to the simplest form of vibrations. 



If we ask now to what end a tense and elastic membrane is interposed in the 

 passage of the waves of sound, why these are not propagated as air-waves to 

 the perceptive apparatus of the auditory nerves, the answer is simply as follows : 

 The ends of these nerves are suspended in a fluid, and for obvious reasons must 

 be so ; hence the question first concerns the communication of the waves to this 

 fluid. Did the air- wave immediately strike the fluid the transmitted impressioii 

 would be extremely feeble, so that weak or remote vibrations, which, under the 

 existing arrangement we hear distinctly, would be no longer perceptible. Be- 

 sides, it is not easy to see how such a direct transmission of the waves of sound 

 from the air to the fluid could be effected, since a free surface accessible to the 

 external air would be incompatible with the safety of the tender nerves. It is 

 imperative, therefore, that the fluid be enclosed within solid walls ; but walls of 

 osseous structure take up the vibrations of air with as much difficulty as fluids, 

 and would transmit them in the same weakened condition ; hence there was 

 needed some other and suitable medium between the air and the aqueous secre- 

 tion. Now this purpose is fulfilled in the most perfect manner by the system 

 of small bones appertaining to the tympanum. Air-waves are with difficulty 

 transmitted immediately to solid bodies, but easily and with intensity to stretched 

 membrane ; and when, in this way, the membrane has been thrown into oscil- 

 latory movements, as above described, these movements are readily communi- 

 cable to solid bodies — a fact of which every one may satisfy himself by the fol- 

 lowing simple experiments : Over the opening of a glass or a tube let a dry 

 membrane be stretched, whether it be of bladder, leather, or mere paper, and let 

 this membrane be strewn with fine sand ; if a tuning fork, which has been made 

 to vibrate by a strong blow, be held at a little distance above the membrane, 

 the lively vibrations into which the membrane is thrown by the air waves pro- 

 ceeding from the fork will be manifested by a saltatory movement of the grains 

 of sand as they continue to bound upwards and fall again on the vibrating sui'- 

 face. To show the fiicility wnth which vibrations of the membrane are trans- 

 mitted to solid bodies, let a metallic ring, such, for instance, as is used for keys, 

 be clasped round with the thumb and forefinger of the left hand, while, with the 

 right a strongly vibrating tuning fork is held close above it. With the simple 

 ring not the slightest motion will be felt, but if a membrane be previously 

 stretched upon it and the experiment be repeated in the same way, the ring will 

 be felt to vibrate in the most sensible manner as long as the vibrating fork con- 

 tinues in its vicinity. 



We now know how the air- waves are received by the membrane of the 

 tympanum ; let us next see how this membrane communicates its vibrations to 

 the chain of small bones behind it, and in what way these in turn set in undu- 



