EFFECTS OF COUPLING 275 



When the coupling became smaller, the characteristic not only developed 

 a more asymmetrical shape but also shifted its operating point A more 

 and more from its normal position A . This departure from linearity on 

 shift is due to the interplay of the lipped edges upon which the malleus- 

 incus joint moves with change in stiffness of the capsular ligament of the 

 malleus-incus junction. Since the effective forces which produce the sep- 

 aration between malleus and incus are the interaural tensors and the 

 elastic reactions of the capsular ligaments surrounding the joint, 

 the asymmetrical non-linear transmission of the wave forms through 

 the malleus-incus articulation must be progressively emphasized as the 

 effective tension on the malleus-incus joint is increased (smaller 

 coupling) . 



Evidence to support such a view can be found in the experimental 

 work of Crowe [1932], who found that, when the tensor tympani of a cat 

 was put under mechanical tension, as the load increased from 5 to 20 

 grams, the low-frequency sounds were more and more suppressed. 

 Finally, for a load of 50 grams only frequencies above 2048 cycles were 

 transmitted. This information confirmed many of the earlier experi- 

 ments which indicated that, the greater the contraction of this tensor, 

 the higher the pitch of the sounds heard. 



If increased tension of the tensor tympani suppresses low-frequency 

 reception, it should follow that decreased tension as obtained by cutting 

 the tympanic tensor should favor low-frequency transmission. Bunch 

 and Raiford [1931] showed experimentally that loss of transmission of 

 high tones resulted from the cutting of the tensor tympani. 



A change in coupling is also produced by a change in pressure of the 

 external air. A temporary low-tone deafness is experienced when one 

 rapidly descends in an elevator or enters a long vehicular tunnel which is 

 subjected to pressure ventilation. Under these circumstances the 

 tympanic membrane is first driven inward by the higher pressure, as if 

 an increase in tension of the tympanic tensor had taken place, which 

 produces low-tone deafness. Relief is afforded by swallowing, which 

 opens the Eustachian tube and allows the pressure in the middle ear to 

 increase to that prevailing in the outer ear canal. 



The two interaural muscles normally function in opposition so that at 

 various acoustic intensities the effective coupling of the malleus-incus 

 joint is changed. The cutting of the tympanic tensor must produce an 

 operating malleus-incus characteristic that is very asymmetrical and 

 even very non-linear for low intensities. Figure VII-13^4. and B shows 

 the progressive changes that may be expected in the output of a linear 

 response mechanism as the slope of the characteristic changes with 

 progressively smaller coupling. When the slope is 45° the impressed 



