626 THE SENSES 



set in the transverse dimension. In support of this assumption 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 frequency 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 corresponding 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 

 sounds, but to render them perceptible, and which is similar in construction 

 to artificial musical instruments, but which far surpasses them in the delicacy 

 as well as the simplicity of its execution. For, while in a piano every string 

 must have a separate hammer by means of which it is sounded, the ear pos- 

 sesses a single hammer of an ingenious form in its ear bones, which can make 

 every string of the organ of Corti sound separately." 



Auditory Judgments. Direction. The power of perceiving the 

 direction of sounds is not a faculty of the sense of hearing itself, but is an act 

 of the mind judging on experience previously acquired. From the modifica- 

 tions which the sensation of sound undergoes according to the direction in 

 which the sound reaches us, the mind infers the position of the sounding 

 body. The only true guide for this inference is the more intense action of the 



