CH. LVI.] PHYSIOLOGY OF HEAKING 795 



In connection with the external ear there is not much more to be 

 said ; the pinna in many animals is large and acts as a kind of natural 

 ear-trumpet to collect the vibrations of the air ; in man this function 

 is to a very great extent lost, and though there are muscles present to 

 move it into appropriate postures, they are not 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 con- 

 sidered, namely, the action of the membrana tympani, 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 many octaves. This would clearly be impos- 

 sible 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 con- 

 tinuance 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, an 

 interchange of gases takes place between the imprisoned air and the 

 blood of the tympanic vessels. In time, as in the aerotonometer 

 (see p. 363), equilibrium is established and the tension of the 

 imprisoned gases becomes equal to that of the blood-gases, not to 

 that of the atmosphere. The membrane is therefore cupped inwards 

 by the atmospheric pressure on its exterior ; 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 deaf- 

 ness, which sensations are at once relieved by swallowing, so 

 as to open the Eustachian tube and thus re-establish equality of 

 pressure. 



The ossicles communicate the vibrations of the membrana 

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

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



