USES OF THE MEMBRANA TYMPANI. 751 



tension, and vibrates under the influence of the waves of sound. Attached 

 to this membrane, is the angular chain of bones, which conducts its vibra- 

 tions, like the bridge of a violin, to the liquid of the labyrinth. The mem- 

 brane is fixed at its periphery and has air upon both sides, so that it is under 

 favorable conditions 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 varia- 

 tions in tension, due to the contraction of the tensor tympani. It is also evi- 

 dent that this membrane may be drawn in and rendered tense by exhausting 

 or rarefying the air in the drum. If the mouth and nose be closed and an 

 attempt be made to breathe forcibly by expanding the chest, the external 

 pressure tightens the membrane. In this condition the ear is rendered in- 

 sensible to grave sounds, but high-pitched sounds appear to -be more intense. 

 If the tension be removed, as may be done by an act of swallowing, the grave 

 sounds are heard with normal distinctness. This experiment, tried at a con- 

 cert, produces the curious effect of abolishing 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 muscle into action, 

 the limit of the perception of high tones is greatly increased. In two in- 

 stances of this kind, recorded by Blake, the ordinary limit of perception was 

 found to be three thousand single vibrations, and by contraction of the mus- 

 cle, this was increased to five thousand single vibrations. 



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 tym- 

 pani. Nevertheless the membrana tympani repeats sounds by influence, and 

 it 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 mem- 

 branes 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 by a membrane, and the tone with which it happened 

 to be in unison would overpower all other tones. 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 fundamental, which would 

 produce errors and confusion in auditory appreciation. The chain of bones, 

 also, attached to the membrane, acts as a damper and prevents the persist- 



