CH. Lin.] PHYSIOLOGY OF HEARING. 7 2 3 



under the control of the will in the majority of people, and are 

 functionless, ancestral vestiges. 



In the middle ear, however, there are several points to be 

 considered, namely, the action of the membrana tympaui, of the 

 ossicles, of the tympanic muscles, and of the Eustachian tube. 



The Membrana Tympani. This membrane, unlike that of 

 ordinary drums, can take up and vibrate in response to, not only 

 its own fundamental tone, but to an immense range of tones 

 differing from each other by as much as seven octaves. This 

 would clearly be impossible if it were an evenly stretched 

 membrane. It is not evenly nor very tightly stretched, but owing 

 to its attachment to the chain of ossicles it is slightly funnel- 

 shaped : the ossicles also damp the continuance of the vibrations. 



When the membrane gets too tightly stretched, by increase or 

 decrease of the pressure of the air in the tympanum, then the 

 sense of hearing is dulled. The pressure in the tympanic cavity 

 is kept the same as that of the atmosphere by the Eustachian 

 tube, which leads from the cavity to the pharynx and so to the 

 external air. The Eustachian tube is not, however, always open; 

 it is opened by the action of the tensor palati during swallowing. 

 Suppose it were closed owing to swelling of its mucous membrane 

 this often happens in inflammation of the throat the result 

 would be what is called Eustachian or throat deafness, and this is 

 relieved by passing a catheter so as to open the tube. When the 

 tube is closed, the blood in the vessels of the tympanic wall takes 

 up oxygen from the imprisoned air, and gives off carbonic acid in 

 exchange ; but the amount of carbonic acid given out is less than 

 the amount of oxygen removed, so that the total quantity of gases 

 within the tympanum is reduced, and its pressure consequently 

 becomes less than that of the atmosphere, so the membrane is 

 cupped inwards ; it is this increased tightening of the membrane 

 that produces deafness. There is also an accumulation of mucus. 

 When one makes a violent expiration, as in sneezing, some air is 

 often forced through the Eustachian tube into the tympanum. 

 The ears feel as though they were bulged out, as indeed the 

 membrana tympani is, and there is again partial deafness, which 

 sensations are at once relieved by swallowing so as to open the 

 Eustachian tube and so re-establish equality of pressure once more. 



The ossicles communicate the vibrations of the membrana 

 tympani (to which the handle of the malleus is fixed) to the 

 membrane which closes the fenestra ovalis (to which the foot of 

 the stapes is attached). Thus the vibrations are communicated 

 to the fluid of the internal ear which is situated on the other side 

 of the oval window. 



3 A 2 



