THE MEMBRANA TYMPANI 737 



labyrinth. The membrane is fixed at its periphery and has air on both 

 sides, so that it is under conditions favorable for vibration. 



A study of the mechanism of the ossicles and muscles of the middle 

 ear shows that the membrana tympani is subject to certain physiological 

 variations in tension, due to the contraction of the tensor tympani. It is 

 also evident that this membrane may be drawn in and rendered tense by 

 exhausting or rarefying the air in the drum. If the mouth and nose are 

 closed and an attempt is made to breathe forcibly by expanding the chest, 

 the external pressure tightens the membrane. In this condition the ear 

 is rendered insensible to grave sounds, but high-pitched sounds appear 

 to be more intense. If the tension is removed, as may be done by an 

 act of swallowing, the grave sounds are heard with normal distinctness. 

 This experiment, tried at a concert, produces the curious effect of abol- 

 ishing a great number of the lowest tones, while the shrill sounds are 

 heard very acutely. The same phenomena are observed when the 

 external pressure is increased by descent in a diving bell. 



Undoubted cases of voluntary contraction of the tensor tympani 

 have been observed by otologists ; and in these, by bringing this mus- 

 cle into action, the limit of perception of high tones may be greatly in- 

 creased. In two instances of this kind, recorded by Blake, the ordinary 

 limit of perception was found to be three thousand single vibrations, and 

 by contraction of the muscle, this was increased to five thousand. 



The concave form of the membrana tympani and the presence of a 

 bony process between its layers, which is part of the chain of bones of 

 the middle ear, are conditions under which it is impossible that it should 

 have a single fundamental tone. This has been shown by experiments 

 with stretched membranes depressed in their central portion by means 

 of a solid rod. No membrane can have a single fundamental tone 

 unless it be in a condition of uniform tension, like a string, and this is 

 impossible in the membrana tympani. Nevertheless, the membrana 

 tympani repeats sounds by influence and is capable of repeating in this 

 way a much greater variety of sounds than if it had itself a fundamental 

 tone and were capable of a uniform degree of tension. This has been 

 shown by experiments with stretched elastic membranes made to assume 

 a concave form. If the membrana tympani had a single fundamental 

 tone, it would vibrate by influence only with certain tones in unison with 

 it and the overtones would be eliminated. It would then act like a 

 resonator closed with a membrane ; and the tone with which it hap- 

 pened to be in unison would overpower all others. The fact is that all 

 tones, the vibrations of which reach the membrane, are appreciated 

 at their proper value as regards intensity. Again, if the membrana 

 tympani had its own fundamental tone, it would have overtones of the 



