660 THE SENSES 



and forty thousand per second, respectively, but only a small number of 

 tones can be perceived outside of the narrower limits given above. This 

 extraordinary range of tone is conceivable only on the supposition of local- 

 ization of the stimulus in some part of the organ. Most physiologists look 

 to the basilar membrane and the organ of Corti for the localization. 



Suppose a simple tuning-fork to be vibrating with a frequency of sixty- 

 four per second, then these waves will be conducted through the auditory 

 apparatus until they fall on the basilar membrane, and will set it in vibration 

 at the same rate. The exact type of the vibration is at present a matter of 

 inference. The piano theory of Helmholtz gives probably the most satisfac- 

 tory explanation. It assumes that the basilar membrane vibrates as would 

 a number of strings set in its transverse dimension. In support of this as- 

 sumption it is asserted that the membrane is taut in the transverse and 

 loose in the longitudinal plane. Retzius has estimated that it contains 

 about 24,000 fibers, and that it measures in width at the base 0.135 mm. 

 and at the apex 0.234 mm. In the above illustration the vibration fre- 

 quency of sixty-four would supposedly set in sympathetic vibration that part 

 of the apex of the basilar membrane which vibrated in the same frequency, 

 and the sensory cells of the organ of Corti, located over the vibrating fiber, 

 would be stimulated accordingly. In the same way notes of medium and 

 of high frequency stimulate localized areas of sensory cells in the middle 

 and basal parts of the organ of Corti and produce sensations of correspond- 

 ing pitch. 



This idea of localization of auditory sensory stimulation makes it easier 

 to understand the analysis by the ear of compound sonorous waves. Such 

 waves impinge on the membrana tympani and are transmitted through the 

 conducting media unanalyzed, and may be supposed to fall on the basilar 

 membrane as compound waves. The basilar fibers acting like so many 

 resonators, take up the constituent sonorous elements in sympathetic vibra- 

 tion. In short, the basilar membrane is an analyzer in which the compound 

 wave is reduced to its simple components, each of which stimulates its cor- 

 responding portion of the organ of Corti. The auditory nerve impulses are 

 conducted through the cochlear nerves to the sensorium where they produce 

 auditory sensations with the same definiteness of pattern as cutaneous or 

 optical stimuli produce sensations that correspond to the patterns of stimula- 

 tion. The audition is so definite that one can consciously pick out one or 

 the other of the constituent stimulating elements and follow and examine 

 the same to the exclusion of the others, as when one follows a single instru- 

 ment in an orchestra or a single voice in a group of chattering children. 



Bernstein says of this wonderful organ: 



"In the cochlea we have to do with a series of apparatus adapted for per- 

 forming sympathetic vibrations with wonderful exactness. We have here 

 before us a musical instrument which is designed not to create musical 



