570 



PHYSIOLOGY 



are thence transmitted by a chain of three small bones, the auditory 

 ossicles, across the cavity of the tympanum to the fluid which bathes 

 the terminations of the auditory nerve in the internal ear. Since the 

 drum of the ear has to pick up and transmit vibrations of every 

 frequency, and to reproduce accurately in its movement the finest 

 variations of pressure in the course of the wave, it is essential that 

 it should be devoid of any periodicity, i.e. a tendency to vibrate at a 



11 



FIG. 246. Diagrammatic view of auditory organ. (After SCHAFER.) 

 1, auditory nerve ; 2, internal auditory meatus ; 3, utricle ; 5, saccule ; 



6, canalis media of cochlea ; 9, vestibule containing perilymph ; 12, stapes ; 



13, fenestra rotunda ; 19, incus ; 18, malleus ; 17, membrana tympani ; 



16, external auditory meatus ; 14, pinna of external ear ; 23, Eusta- 



chian tube. 



certain frequency. If such periodicity were present the ear would 

 pick out and magnify some particular overtone present in the com- 

 pound tones reaching the ear and magnify it to the exclusion of the 

 other overtones. The perfect aperiodicity of the tympanic membrane 

 is secured by its structure and attachments. The membrane is 

 composed of a thin layer of fibrous tissue covered externally with skin 

 and internally by the mucous membrane of the tympanum. To its 

 inner surface is attached the handle of the malleus, the first of the 

 auditory ossicles, along its whole length. This attachment of an 

 elastic membrane to a mass of bone would itself tend to damp any 

 vibrations of the membrane. By the attachment of the tendon of 



