Il8 HEARING. 



With reference to the transmission of sound, the cavity 

 of the vestibule and cochlea may be regarded as divided 

 into two, that of the vestibule and scala vestibuli 

 which communicates with the lymph space surrounding 

 the vestibular sacs, and that of the scala tympani, which 

 is- closed by the membrane of the fenestra ovalis. 

 Although these two cavities communicate by a small 

 opening, they may be regarded as in so far separate that 

 any motion of the membrane of the foramen ovale must 

 be communicated to the membrane of Reissner, then to 

 the membrana basilaris and thereby to the organ of Corti. 

 From the structure of the organ of Corti it was inferred 

 by Helmholtz that it must be an organ for the perception 

 and discrimination of tones, and that the elements (nerve- 

 endings, Corti's arches and adjoining cells) serve in the 

 discrimination of the sonorous vibrations of the liquid in 

 which they are immersed a function which is analogous 

 to that of the hypothetical red, blue, and green elements 

 of the retinal cones in regard to luminous vibrations. 

 Tones can be readily discriminated by the human ear of 

 which the vibration-rates range from 40 to 7,000 per second. 

 A skilled musical ear can distinguish more than 6,000 

 different tones within the range of these seven and a half 

 octaves ; consequently, as there are only 3,000 arches of 

 Corti, each must be capable of being excited by several 

 gradations of tone. 



In the case of the elements of the cones of the retina 

 we can form no conception of the modes in which they 

 are acted upon by light, but the action of tones on the 

 organ of Corti can be satisfactorily explained by com- 

 paring that organ to a system of resonators. By a reso- 

 nator is meant anything which, by virtue of its form and 

 structure, is capable of being thrown into musical vibra- 

 tion. Every resonator produces when thus acted upon, a 

 tone which is peculiar to itself its " proper tone," and is 

 readily excited by tones of the same vibration- rate when 



