122 Mr. R. Moon on the Structure of the Human Ear, and on 



For under such circumstances, if a wave of rarefaction, for in- 

 stance, were incident upon the tympanum, the pressure of the 

 air without the tympanal membrane being less than the mean 

 pressure, while the air within the tympanal membrane has the 

 mean pressure, a motion of the tympanal membrane — which (if 

 any) would necessarily be a motion outwards — could only take 

 place by reason of the membrane being stretched. The occur- 

 rence of such a motion outwards would afford decisive proof that 

 the membrane was capable of being stretched; and, being so 

 capable, it would follow, when a wave of condensation was inci- 

 dent upon it (the external being in this case greater than the in- 

 ternal pressure), that motion of the tympanal membrane would 

 again occur, though in this case taking place in a direction con- 

 trary to that in which it occurred in the former. 



But the tympanal membrane is neither flat, nor are its move- 

 ments confined simply by the quasi-circular tympanal bone to 

 which it is affixed. 



The membrane is concave outwards, convex inwards; from 

 which it results, as will immediately be shown, that the action 

 upon it of rarefied waves and of condensed waves must be radi- 

 cally different. 



When a rarefied wave is incident on the membrane, the motion 

 will take place outwards ; and the membrane being concave out- 

 wards, all that is requisite for this is a simple flexure, a simple 

 change of form of the membrane without any stretching, and 

 which may be effected whether the membrane be elastic, or ca- 

 pable of being stretched, or not. 



When a condensed wave is incident upon the membrane, on 

 the other hand, the circumstances are altogether different. The 

 motion in this case (if any) must take place inwards ; and the 

 membrane, being convex inwards, will be incapable of motion 

 unless it be capable of being stretched. Nor would a mere ca- 

 pacity for being stretched be sufficient to allow of continuous 

 action of the ear for auditory purposes. The membrane must 

 possess the power of speedily returning to its original status ; i. e. 

 it must be highly elastic. 



When the ear is in its normal state, therefore, it clearly ap- 

 pears that, in order to the transmission to the sensorium of the 

 vibrations of a rarefied wave, flexibility of the tympanal mem- 

 brane without elasticity is sufficient; while for the like trans- 

 mission of the vibrations of condensed waves elasticity of the 

 membrane is essential. 



What, then, is the character as regards elasticity of the tym- 

 panal membrane ? The membrane is thus described by the late 

 Mr. Toynbee :— 



"Looked at from without inwards, the membrana tympani 



