HEARING 653 



is now recognized that muscial tones and noises are not fundamentally 

 different, and that both are perceived by the cochlea, the vestibular 

 apparatus having nothing to do with hearing. 



f- On embryological grounds it does not seem probable that a 

 membrane of mesoblastic origin would stimulate hair cells of 

 epiblastic origin. It has also been pointed out that in some 

 animals for example, the pig the basal portion of the organ of 

 Corti rests upon a bony plate which takes the place of the basilar 

 membrane. 



To meet these objections the view that the tectorial membrane 

 vibrates has been put forward by several observers. As originally 

 put forward, it assumed that high-pitched tones caused a vibration 

 of the basal end of the membrane, and in descending the scale 

 more and more of the membrane was set in vibration, until 

 with the lowest tones the whole membrane vibrated. Such an 

 explanation fails to account for the ability of the musician's 

 ear to perceive separately the different sounds from an orchestra 

 which reach it simultaneously, and, furthermore, does not account 

 for partial defects " tone-gaps " in the range of hearing. The 

 view has therefore been modified on the lines of the Helmholtz view, 

 and it is now suggested that the tectorial membrane vibrates to 

 different tones in different parts of the cochlea in the same manner 

 as suggested for the basilar membrane. It is pointed out that the 

 tectorial membrane gradually increases in breadth from base to apex 

 after the manner of the basilar membrane, and that, by virtue of 

 its elasticity, transverse flexibility, and the fact that it is attached 

 on one side only, it makes a better and more sensitive vibrator than 

 does the basilar membrane. 



Auditory Judgments. In arriving at auditory judgments, we are 

 aided by other senses and by knowledge previously acquired. Nor- 

 mally, we refer sounds to the exterior of the body, and from the nature 

 of the sound determine what it is, its direction, distance, and so forth. 

 According to its quality and loudness, we pronounce a sound to be a 

 gunshot fired in a certain direction, close at hand, or far away. The 

 direction of a sound is determined largely by the force with which 

 it strikes the two ears. Judgment of direction is aided by turning 

 the head from side to side, so that first one ear and then the other 

 receives the sound fully. It is easier to judge the distance of noises 

 than of musical sounds. The power to judge the distance of the source 

 of sound depends on previous experience. Therein lies the effect 

 of distance imparted by the operatic chorus, which, on leaving the 

 stage, by singing more and more softly, gives the impression of passing 

 farther and farther into the distance. 



