246 The Structure of the Cochlea and the Perception of Tone 
especially how certain cells are stimulated by impulses coming from 
particular tones only and not from others; these are problems in the 
resonator theory which occupied the attention of Helmholtz and have 
been the subject of investigation by the physiologists who have since 
made a study of the question of tone perception. 
The structure of the hair cells themselves suggests the probable way 
in which a stimulus is applied to these cells. Hach cell is provided with 
a clump of short hairs which project from its free end. The stimulation 
of these cells is obtained presumably when these hairs are brushed against 
the overhanging membrana tectoria. 
The difficult problem in the resonator theory is to determine how cer- 
tain hair cells are stimulated by certain tones only while other cells are 
stimulated by other tones. Helmholtz was at first inclined to attribute 
to the rods of Corti this selective function of resonators. When it was 
shown that birds and crocodiles lack these rods, Helmholtz gave up this 
idea and fixed upon the radiating fibers of the membrana basilaris as the 
real resonators of the organ of Corti. These fibers, estimated from 
15,000 to 25,000 in number and varying in length from 0.04 mm. in 
the basal coil to 0.5 mm. at the apex of the cochlea, should vibrate, accord- 
ing to Helmholtz, each for a particular tone. In vibrating, the superim- 
posed hair cells presumably would be carried upwards and their project- 
ing hairs brushed against the membrana tectoria. 
Objections to this view of the function of the membrana basilaris have 
been expressed. In the first place, the tone differences that can be 
recognized by different individuals are by no means the constant factors 
we would expect to find in case a distinct anatomical entity, a separate 
fiber of the basilar membrane, was endowed with the ability to 
vibrate each in response to a distinct tone. A trained musical ear is 
capable of recognizing a much smaller difference between tones than an 
untrained ear. Some, at least, of the difficulties in his theory were ap- 
preciated by Helmholtz himself as, for example, the question whether it 
is possible for fibers as short as those of the basilar membrane to be 
thrown into vibration at all by sound waves. Ewald recently, in an 
ingeniously constructed ear model, has been able to demonstrate that a 
stretched rubber membrane, measuring 0.5 mm. broad, could be made to 
vibrate, when suspended in water, by sound waves conducted from the 
air. Such a demonstration, however, falls far short of proving that the 
much shorter, thicker, and more rigid membrana basilaris could respond 
ina similar manner. It must not be lost sight of in this connection that 
the cells forming the organ of Corti are not placed separately on the 
