652 THE SENSES. 



The stapes accordingly forms a kind of movable lid or piston-head 

 occupying the orifice of the fenestra ovalis, and capable of transmitting 

 directly to the fluid of the labyrinth the impulses received from the 

 membrana tympani. The extent of inward and outward movement of 

 the base or footpiece of the stapes has been determined by Helmholtz 

 in the following manner. The cavity of the tympanum and that of the 

 vestibule having both been opened from above, the point of a line sewing 

 needle was inserted into the fibrous covering of the base of the stapes 

 from the side of the vestibule, and the needle allowed to rest, near the 

 point of its'insertion, upon an adjacent edge of bone. It thus formed a 

 kind of index-lever, which would indicate by the displacements of its 

 long arm, very slight movements of the stapes. The stapes was then 

 pressed inward and outward, as freely as its attachments would allow, 

 either by means of a needle applied to the bone itself, or by alternately 

 condensing and rarefying the air in the external auditory meatus ; the 

 force, in the latter case, being transmitted through the membrana t3'm- 

 pani and chain of bones. The same observer estimated these movements 

 according to another plan, by opening the superior semicircular canal 

 of the labyrinth, and inserting into it a slender glass tube of known 

 calibre, a portion of which, as well as the cavity of the vestibule, was 

 filled with water. Any inward pressure upon the stapes would accord- 

 ingly be indicated by a corresponding rise of the level of water in the 

 tube. The movement of the stapes, in these experiments, varied, accord- 

 ing to circumstances, from .025 to .072 millimetre. 



The change of position of the stapes in the fenestra ovalis, due to the 

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

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

 upper border is tilted over toward the cavity of the vestibule and back 

 again, and its anterior end moves more freely than its posterior. This 

 feature of the action of the stapes, which has been described by several 

 observers, is shown by Helmholtz to depend upon the varying compact- 

 ness of its fibrous attachments ; these attachments being closer along 

 its inferior border and at its posterior end, thus allowing more freedom 

 of movement above and in front than below and behind. 



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 cavity of the tympanum ; and its slender tendon, 

 after entering this cavity, passes almost directly forward and is inserted 

 into the posterior side of the neck of the stapes, near its articulation 

 with the incus. Its contraction will, therefore, draw the angle of the 

 stapes backward and its anterior extremity outward from the fenestra 

 ovalis. 



Physiological Action of the Chain of Bones and the Muscles of the 

 Middle Ear. The cavity of the tympanum is an irregularly shaped 

 space, inside the membrana tympani, filled with air, across which the 

 vibrations received by the membrane from without are transmitted by 

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



