ACTION OF THE EAR MUSCLES. 365 



It is to be remarked that the membrana tympani is placed obliquely 

 at the bottom of the external canal. In a hollow bony cone, Action of the 

 rising upon the interior wall of the tympanum, and called the tensor'tym-* 

 pyramid, the stapedius muscle is placed. Through a little pani. 

 aperture at the apex of the pyramid its tendon goes out, and is inserted 

 in the neck of the stapes. Its action seems to be to make pressure on 

 the membrane of the fenestra ovalis, but as it does this, it tilts the stapes 

 into an oblique position. A second muscle, the Censor tympani, is at- 

 tached in front to the under surface of the petrous bone, and is inserted 

 in the short process of the malleus ; when it contracts it makes tension 

 upon the membrana tympani, drawing it more tightly inward. It is to 

 be especially remarked of both these muscles that they are voluntary ; 

 that is, of the striated variety. Two other muscles are described by 

 some anatomists, and have been indicated in Fig. 183 and 184. Their 

 existence, however, is disputed by others. 



In the opinion of Mr. Toynbee, the action of the two voluntary mus- 

 cles of the ear is as follows. By the tensor tympani the Mr.Toynbee's 

 base of the stapes is pressed inward to ward the vestibule, as views of the ac- 



_. r n _ , , tionof the sta- 



is a piston in its cylinder, and, as soon as the muscle ceases pediusandten- 

 to act, the elastic ligaments which attach the circumference sor ^P* 111 - 

 of the base of the stapes to that of the fenestra ovalis draw it out again. 

 The stapes is moved by two muscles, the tensor tympani and the stape- 

 dius, it being commonly supposed that the latter. aids the former in press- 

 ing the stapes inward, but he shows that it rotates' the base of the stapes 

 and withdraws it from the cavity of the vestibule. This may be demon- 

 strated by pulling the stapediusj when the fluid in the scala vestibuli 

 will be found to move correspondingly. He therefore asserts that the 

 stapedius is the antagonist of the tensor, the former relaxing the laby- 

 rinthine fluid, membrana rotunda, and membrana tympani, the latter 

 rendering them more tense. Agreeably to this, the stapedius is supplied 

 from the portio dura, and the tensor from the otic ganglion. This con- 

 struction might lead to the supposition that the tensor affords protection 

 from loud sounds, and the stapedius enables the most delicate whisper to 

 be heard, as in listening. Together they regulate the amount of sono- 

 rous vibrations which enter the labyrinth. Hence the effect of the de- 

 struction of the membrana tympani is to make sounds unendurable. In 

 confirmation of this is quoted the case of a patient who, under those cir- 

 cumstances, could not bear the whistling of another patient in an adjoin- 

 ing bed, and the observation of Cheselden that a dog, in which both 

 membrana3 tympani had been destroyed, for some time received strong 

 sounds with horror. 



We shall now present some reasons for supposing that the function 

 of the tympanum is for determining the first property of sounding waves, 

 that is, their intensity. 



