204 PHYSIOLOGY CHAP. 



resonator that is tuned to every note of the scale. Observation 

 shows, indeed, that under normal conditions our ear is capable of 

 perceiving, with almost equal accuracy, all the notes of a scale 

 that extends from 30 to 4000 vibrations per second. In account- 

 ing for this fact, Helmholtz laid great stress on the funnel-shaped 

 form of the membrana tympani, owing to which there is unequal 

 tension at its central or more or less peripheral parts, as well as 

 on its load, formed by the auditory ossicles which damp its after- 

 vibrations in the same manner as the contrivance used on the 

 piano for damping the vibrations of the strings. According to 

 Helmholtz the ear contains a very perfect apparatus for damping : 

 we are able to distinguish about ten different notes per second. 

 This rapid damping evidently depends not only on the chain of 

 ossicles, but also on the endolymph. 



Hensen, in developing Helmholtz' theory, assumed that the 

 tympanic membrane may from its special structure be regarded as 

 a resonator producing an infinite number of proper tones, like the 

 membrane of Hensen's phonautograph, which is curved like the 

 tympanum, and is fixed in one of its radii by a solid body corre- 

 sponding to the malleus. 



Fick, too, attempted to imitate the tympanic apparatus by 

 artificial models, and found them capable of reproducing and re- 

 inforcing a multiplicity of sounds and tones. He hoped by these 

 experiments to elucidate the mechanism of the tympanum, 

 starting from the hypothesis that it is capable, when affected by 

 different tones, of vibrating separately in its different sectors, as 

 though composed of a corresponding number of strings of different 

 length and tension. But, as Hensen correctly observes, the 

 tympanum is so firmly woven together that it is impossible to 

 admit the isolated movement of its respective radial fibres. 



The ability of the tympanic membrane to vibrate in unison 

 with the different musical notes is consequently still unexplained. 

 We can construct mechanical contrivances which have approxi- 

 mately the same power the resonance boxes of musical instru- 

 ments, which reinforce all sounds, being the best example but no 

 mechanical theory can at present be put forward. 



The movement of great amplitude and limited force, writes 

 Helmholtz, which reaches the tympanic membrane from the 

 external air in the form of a sound-wave, is transformed by the 

 tympanic apparatus into a wave of limited amplitude but greater 

 force, and transmitted to the perilymph. The end of the malleus 

 represents the longest arm of the lever, and the pressure of the 

 stapes is one and a half times greater than the force which 

 presses the end of the manubrium inwards. On transmitting the 

 movement of the incus to the stapes, there is a further consider- 

 able reduction in the amplitude of the vibration, with a corre- 

 sponding increment of force. The stapes presses on the fenestra 



