THE SENSE OF HEARING. 



01 



malleus, which follows its movements, causes its vibrations to meet 

 with considerable resistance. This diminishes the intensity of the 

 vibrations, and prevents also the continued vibration of the mem- 

 brane after an external vibration has ceased, so that a sound is not 

 heard much longer than the moment when the exciting cause ceases. 

 The tensor tympani at its base arises from the apex of the petrous 

 >rtion of the temporal bone and the cartilage of the Eustachian 

 tube, and is inserted into the malleus near its root. The membrana 

 tympani has the handle of the malleus inserted into its layers; and 

 is the malleus and incus move around an axis passing through the 

 icck of the malleus from before backward, the action of the tensor 

 tympani is to pull the membrana tympani inwards toward the 

 tympanic cavity in the form of a funnel, the meridians of which 

 are not straight, but curved with the convexity outwards. This 

 making tense and relaxing the membrana tympani is a kind of 

 accommodating apparatus for receiving and transmitting sounds 

 of different pitch. With different tensions it will respond more 

 readily to sounds of one pitch than to sounds of another. 



The tensor tympani receives its motor fibers from the fifth by 

 the otic ganglion, and its movements are purely reflex. 



The stapedius muscle is innervated by the facial and exercises 

 an antagonistic action to the tensor tympani. The stapedius draws 

 the stapes outward, whilst the tensor tympani tends to press the 

 stapes into the oval window. The two antagonistic muscles are able 

 to combine in such a manner as to modify the length of the chain 

 of ossicles, and give an amplitude variable to the vibrations. 



Hensen has proved that these muscles are reflexly kept in a state 

 of adjustment by the pitch of sound. 



Transmission of Sound Waves in the Middle Ear. The normal 

 and regular means of transmission of sounds is by the chain of ossi- 

 cles, but it can take place by the air in the cavity of the tympanum, 

 or by the bones of the skull. 



The ossicles of the middle ear form, by the articulations which 

 mite them, a broken but rigid chain between the membrana tympani 

 and the oval window. This chain of bones is not always in the same 

 state, for the combined action of the two muscles modifies the length 

 and rigidity of the chain. Pollitzer, by means of very fine pens, has 

 been able to register the movements of the bones. In rarefaction 

 the air in the auditory meatus, as with the pneumatic speculum, 

 there is no danger of pulling the stapes out of the oval window, for 

 le incus only follows the malleus for a certain distance, the latter 



