708 THE SENSES 



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 to only those tones whose vibrations 



B 



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

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



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

 tympanic membrane can vibrate in response to a wide range of tones, thus 

 transmitting vibration frequencies ranging between 30 to 4,000 or 5,000 

 per second, or even higher. The limits of hearing as regards pitch are in 

 the ordinary average ear represented by tones of these vibration fre- 

 quencies. In some individuals the scale extends to 30,000 or more. This 

 range is made possible by the fact that the membrana tympani is by no 

 means under constant tension, but is connected with the chain of auditory 

 ossicles, the contractions of the muscle of which produce variations in ten- 

 sion, especially on the handle of the malleus. If the membrane were quite 

 free in its center, it would be like a drum which goes on vibrating for some 

 time after it is struck, and each sound would be prolonged, leading to con- 

 siderable confusion. This evil is obviated by the ear bones, which check 

 the continuance of the vibrations like the "dampers" in a piano. 



The vibrations of the membrana tympani are transmitted by the chain 

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



