262 PHYSIOLOGY CHAP. 



sensorial perceptions in general are not the reality, but are merely 

 images, or representative signs, of reality. 



XIV. The last point before us is to examine the functional 



importance or utility of binaural audition through which one ear 



is supplemented by the other, which makes the normal auditory 



function more perfect and complete, and facilitates judgment of 



j the direction and distance of the source of the sounds that reach us. 



G. B. Venturi, Professor of Physics at Modeua, was the first 

 who showed that our judgment of the direction of sounds is 

 principally founded on binaural audition, and on the fact that 

 one ear is almost always more strongly excited than the other. 

 He set out his theory in a short Memoir (1802), and illustrated 

 it by four convincing and well-established experiments : 



(a) If a blindfolded person, with one ear stopped and head 

 motionless, is made to listen in the centre of an open space, free 

 from all obstacles, to the tone of a flute, played at a distance of 

 14 or 15 metres, the sound is invariably perceived in the direc- 

 tion of the axis of audition, whatever the position of the person 

 who plays the instrument. By "axis of audition" he means 

 the line vertical to the median plane of the head or external surface 

 of the ear. 



(&) If the subject, instead of standing motionless in the 

 centre of the space, turns slowly round on his own vertical axis, 

 all other experimental conditions remaining unchanged, the tone 

 he hears will be louder when the axis of audition of the open 

 ear approximates to the direction of the sound-waves that reach 

 the ear. Any one who is entirely or partially deaf of one ear has 

 no other means of recognising the direction of a tone than by 

 turning the head and observing how the intensity of the sound 

 alters. But even then they often judge wrongly if the sound 

 only lasts for a moment. 



(c) If both ears are kept open and the subject still has his 

 eyes blindfolded and his head motionless, he is capable of judging 

 the direction of sound with tolerable accuracy, whatever the 

 position of the player, at a distance of 14-15 metres. If the 

 subject puts one finger into the left ear so as to close it gradually 

 he receives the impression that the sound comes from another 

 place, and approaches the axis of audition of the ear that was 

 left open. If the other ear is then gradually unclosed the sound 

 seems to go back to its original, and true, direction. 



(d) With closed eyes, and both ears open, and the head 

 motionless, the subject is unable to distinguish whether the 

 sound comes from before or behind, when its source is in such a 

 direction that both ears are stimulated with the same intensity. 



. " So that it is from the inequality of the two simultaneous 

 sensations of the two ears that we learn the true direction 

 of sound." 



