THE STAPES 277 



characteristic through the origin of coordinates to a change in the 

 transmission frequency of the acoustic wave. 



The degree of coupling of the maleus-incus joint must of necessity be 

 governed by the ability of the muscles to react to the acoustic stimu- 

 lations. Thus, the data gathered by Bunch [1929] and later confirmed 

 by Montgomery [1932] indicate that with increased age the ears lose 

 some of their high-tone responsiveness. At advanced age the trans- 

 mission deafness of high tones is very pronounced, but the loss may be 

 comparatively little for low frequencies. If with increased age the 

 interaural muscles cannot produce the necessary tension, the malleus- 

 incus articulation functions as a close-coupled joint favoring the trans- 

 mission of low frequencies. 



Since the stapedius muscle and the tensor tympani muscle function 

 antagonistically to produce an effective overall tension of the ligaments 

 of the malleus-incus capsule, it follows that if the tendon of the tympanic 

 tensor is cut, so that the effective tension can be produced only by the 

 stapedius muscle, the result would be the production of an operating 

 characteristic with an asymmetric large coupling factor which is favor- 

 able to the passage of low frequencies of high intensities. The experi- 

 ments by Stevens, Davis, and Lurie [1935] on the localization of pitch 

 perception on the basilar membrane, when the tensor tympani muscle 

 had been inactivated, showed that the intense low tones which were 

 transmitted very efficiently were responsible for disrupting the organ 

 of Corti from the aural membrane. 



On the whole the experiments seem to indicate that large coupling of 

 the malleus-incus articulation due to the relaxation of the interaural 

 muscles and consequent close fit of the articulating surfaces is favorable 

 to the passage of low-frequency, low-intensity sounds, whereas con- 

 traction of the interaural muscles, producing a looser-coupled malleus- 

 incus joint, is favorable to high-frequency, high-intensity transmission. 



The Stapes 



The stapes is a stirrup-shaped bone (Fig. VII-14) having a head 

 (capitulum), and an arch formed by the posterior and anterior crura, 

 mounted on a kidney-shaped footplate. The head is connected with the 

 lenticular process at the end of the long cms of the incus, with which 

 it forms a ligament-covered, shallow-socket-and-hemispherical-ball 

 articulation. 



Inserted into the capitulum (C), close to the articulation of the lenticu- 

 lar process of the incus (7) , is a tendon (T) . The stapedius muscle makes 

 its exit at the apex of a bone pyramid and pulls the tendon and attached 

 head backward (posterior) and down (inferior). 



