FUNCTIONS OF THE EXTERNAL AND MIDDLE EARS 657 



internal ear, a larger swing of the basilar membrane, hence a more intense 

 stimulus of the organ of Corti. 



Function of the External and Middle Ears. It has already been 

 stated that the external ear collects the sound waves and conducts them 

 against the membrana tympani. This membrane vibrates as a whole to 

 the compound waves that impinge upon it, and thus serves for the trans- 

 mission of sound from the air to the chain of ossicles of the middle ear. It is 

 often compared to the membrane of a drum, but there are fundamental 

 differences. 



When a drum is struck, a certain definite fundamental tone is elicited; 

 similarly a drum is thrown into vibration when certain tones are sounded in 

 its neighborhood, while it is quite unaffected by others. In other words, it 

 can take up and vibrate in response only to those tones whose vibrations 



B 



FIG. 439. Showing .4 and B, Simple Pendular Vibrations, Separated by One Octave. C, 

 The form of the curve produced by the combination of A and 5. 



nearly correspond in number with those of its own fundamental tone. The 

 tympanic membrane can take up an immense range of tones produced by 

 vibrations ranging from 30 to 4,000 or 5,000 per second. This would be 

 clearly impossible if it were an evenly stretched membrane. The fact is 

 that the membrana tympani is by no means evenly stretched, and this is due 

 partly to its slightly funnel-like form, and partly to its being connected with 

 the chain of auditory ossicles. Further, if the membrane were quite free in 

 its center, it would go on vibrating as a drum does some time after it is struck; 

 each sound would be prolonged, leading to considerable confusion. This 

 evil is obviated by the ear bones, which check the continuance of the vibra- 

 tions like the "dampers" in a piano. 



The vibrations of the membrana tympani are transmitted by the chain 

 of ossicles to the fenestra ovalis and fluid of the labyrinth, their dispersion 

 in the tympanum being prevented by the difficulty of the transition of vibra- 

 tions from solid to gaseous bodies. The necessity of the presence of air on 

 the inner side of the membrana tympani, in order to enable it and the audi- 

 42 



