THE SENSES. 557 



The change of position of the stapes in the fenestra ovalis, from 

 impulses received through the chain of bones, is not a simple move- 

 ment of advance and recession, but a rocking motion, in which its 

 upper border is tilted back and forward. This action of the stapes 

 depends on the varying compactness of its fibrous attachments, which 

 allow more freedom of movement above than below. 



The position of the stapes is also regulated by the action of the 

 stapedius muscle. This muscle, the smallest in the body, arises from 

 a bony canal behind the tympanum ; its slender tendon passing almost 

 directly forward to be inserted into the neck of the stapes, near its 

 articulation with the incus. Its contraction, therefore, draws the 

 angle of the stapes backward, and its anterior extremity outward 

 from the fenestra ovalis. 



Physiological Action of the Bones and Muscles of the Middle Ear. 

 The cavity of the tympanum is an irregularly shaped space, across- 

 which the vibrations received by the membrana tympani are trans- 

 mitted by the chain of bones. In their natural position and with their 

 tendinous connections undisturbed, these bones are in such close con- 

 nection with each other that they vibrate as a single body. 



The action of the internal muscle of the malleus, or tensor tympani r 

 is, no doubt, as its name indicates, to increase the tension of the mem- 

 brana tympani. It has long been known that, after opening the cavity 

 of the tympanum and the canal in which this muscle is lodged, by trac- 

 tion upon its tendon the membrana tympani is rendered more tense ; 

 and, according to Helmholtz, all the ligaments holding the ossicles in 

 place are at the same time put upon the stretch. 



The effect produced upon hearing by increased tension of the mem- 

 brana tympani has been variously interpreted. Savart,* who first 

 studied systematically the vibration of stretched membranes induced 

 by the proximity of sounding bodies, estimated its extent from the 

 agitation of fine sand sprinkled on the membranes ; and found it less 

 pronounced, other things being equal, when the tension of the mem- 

 brane was increased. He applied the same method to the membrana 

 tympani of man and animals, and found that sand, sprinkled on its 

 surface, could be thrown into agitation by holding near it a sounding 

 body, and that these phenomena were less easy of production when the 

 membrane was rendered more tense by traction on the tensor tympani. 

 He concluded that during life the ear is more susceptible to sounds of 

 a given intensity when the membrana tympani is relaxed than when 

 it is on the stretch ; and that the tensor tympani, accordingly, 

 exerts a protective action by lessening the apparent intensity of 

 loud sounds. 



But this observer was not aware of an important fact established 

 by subsequent investigations, namely, that stretched membranes, like 

 cords, cannot respond indiscriminately to sounds of every tone, but 



* Journal de Physiologie. Paris, 1825, tome iv., p. 205. 



