v THE SENSE OF HEAKING 201 



sions of the stapes must, as shown above, be smaller, :md at the 

 same time more powerful than those of the tympanic membrane ; 

 according to Helmholtz and 1'olit/er they do not exceed 0'07 mm., 

 according to Bezold 0'04 mm. 



As the excursions of the chain of ossicles caused by the sound- 

 waves are so small, no change i.ikrs place in the position of tin- 

 articulations. These are only altered by reflex activity of the 

 muscles of the malleus and stapes, when more extensive move- 

 ments of the membrana tympani occur. 



We must now inquire more closely how this apparatus, formed 

 by the tympanic membrane and chain of ossicles, functions as 

 the receiver and transmitter of vibrations. When the sound- 

 waves impinge on it, the tympanic membrane is readily thrown 

 into vibration, on account both of its delicacy and inextensibility 

 and of its low tension, owing to which it offers little resistance. 

 Experiment shows that like the membranes of the telephone 

 and phonograph it vibrates in unison with the different tones of 

 the musical scale, i.e. it follows the vibrations of the air exactly, 

 without reinforcing tones of any given pitch. Politzer (1864) 

 first demonstrated this directly by experiments on the tympanic 

 apparatus of the human dead subject. After opening up the 

 tympanic cavity, he attached to the chain of ossicles, or directly 

 to the surface of the tympanum, a glass thread or straw, the free 

 end of which recorded on a moving drum the excursions produced 

 by tones of different pitch ; these were transmitted to the 

 tympanum from organ pipes, communicating with Helmholtz 

 resonators, which were connected by a rubber tube to the external 

 auditory meatus of the subject. By this method he obtained 

 very definite curves, not only of the simple tones, but also of the 

 compound tones, resulting from the superposition of the different 

 partials. 



Lucae (1864) obtained similar results on conducting the tones 

 through the bones instead of through the external auditory 

 meatus of the dead subject. He thus demonstrated that when 

 we perceive the note of a tuning-fork applied to the bones of 

 the cranium, the effect is due not merely to direct transmission of 

 the waves from the bones to the labyrinth, but also to the inter- 

 vention of the tympanic apparatus. Politzer confirmed this by 

 another ingenious experiment. 



As the lever constitutes a lo;nl which disturbs the vibrations 

 of the ossicles, Buck (1870), under the direction of Helmholtz, 

 made observations of the excursions produced by blowing into 

 the meatus; he observed the excursions directly under the 

 microscope, and measured their amplitude. On illuminating the 

 interior of the tympanum with a strong light, he found that the 

 membrane of the fenestra rotund. i vibrated with the chain of 

 ossicles, and that the excursions of the ossicles persisted after 



