388 ELEMENTARY PHYSIOLOGY less. 



in some way or other affected by the vibrations of the 

 fluids in the cochlea. 



(xii) Localisation of Bound. — The apparatus of the 

 ear which we have described, provides us simply with 

 auditory sensations ; enables us to appreciate high notes 

 and low notes, to discriminate between musical sounds 

 and noises. Experience then enables us to base upon 

 these sensations certain conclusions as to the nature of 

 the source which is giving rise to each sound. The con- 

 clusions wc thus arrive at are usually more or less accurate. 

 But sounds may be coming to us in different directions 

 and fi'om different distances, and when we endeavour to 

 form some estimate of either tlie one or the other of these 

 possible differences we find our means of doing so are very 

 imperfect. As to our estimate of the distance fi'om which 

 a sound is coming, we are guided chiefly by its varying 

 intensity coupled with previous experience and a know- 

 ledge of the laws which connect varying intensity with 

 the different distances of the source. For the discrimina- 

 tion of the direction from which a sound is coming, we have 

 to rely almost entirely on the different effect the sound 

 produces on each of our two ears, according as it falls 

 more directly into one of them than into the other. Thus 

 when we are endeavouring to localise a source of sound, 

 we usually turn the head into various positions until we find 

 one position in which the sound is loudest as it falls into 

 one ear, and then Ave assume that the sound is coming 

 along a line directed straight into that ear. In animals 

 with large and movable external ears, the movement of 

 the ear to a great extent takes the place of the movement 

 of the head ; this may be readily observed in an animal 

 such as the horse. 



Anything which interferes with the ordinary laws of 

 transference of sound causes us to form a wrong judgment 

 as to the distance of the source, as in the case of listening 

 to speech through a telephone or in a phonograph. 

 Similarly, it is difficult to estimate the distance of the 



