THE EAR 287 



The anvil, Jc, has two processes, one behind, Jb, 

 which is movably connected with the posterior wall of the 

 tympanic cavity, and a lower process, Jl, whose point is 

 connected by means of a sesamoid bone with the stapes 

 (stirrup) S. The base (foot) of the stirrup is united with the 

 membrane of the fenestra ovalis (<?, Fig. 38). 



The auditory ossicles form a lever turning- about the axis 

 of the hammer, one of whose arms is the manubrium of the 

 hammer, while the other arm extends from the axis to the 

 point of the lower process of the anvil and, through the 

 stirrup, is connected with the membrane of the fenestra 

 ovalis. If the drum vibrates transversely to and fro, its 

 movements are carried by the lever to the membrane of the 

 fenestra ovalis. 



The sound-conducting apparatus of the middle ear is so 

 constructed that it is evenly set in sympathetic vibration by 

 sound vibrations of various lengths. A free and uniformly 

 stretched membrane gives out, when it is struck, a certain 

 note whose pitch depends upon the size and tension of the 

 membrane. Such a membrane is set in especially strong 

 vibration when in its neighborhood a note having the same 

 pitch as that produced by the membrane is sounded. The 

 drum of the ear has no definite note of its own because of its 

 complicated structure (funnel-shaped, being pulled inward 

 by the manubrium of the hammer). By this its tension in 

 different directions is not the same and therefore it. can have 

 no definite note of its own. It can therefore be set into 

 sympathetic vibration to the same extent by many different 

 notes. 



The sound-conduction apparatus of the ear is provided 

 with a very effective damper, so that no perceptible after- 

 vibrations occur when the notes producing the vibrations 

 have ceased. 



The following muscles are inserted on the auditor}' ossi- 

 cles: 



(i) Tensor tympani, which lies in a bony canal extending 

 parallel with the Kustachian tube. It is united to the 



