Chap, iv.] HEARING. 1023 



the median plane, it is very difficult to say what is its position 

 in that plane, i.e. whether it is more towards the front or back 

 of the head. Hence also a man who is absolutely deaf of one 

 ear has great difficulty in recognizing the direction of sounds. 



Further, when we desire to judge particularly as to the 

 direction of a sound, we listen to it, and in the act move the 

 head into the position in which we hear the sound most dis- 

 tinctly. In this way the movements of the head in hearing 

 play a part somewhat analogous to the movements of the eyes 

 in vision. 



Even in the case of ourselves, and still more in the case of 

 some animals, the form of the external ear favours the entrance 

 into the meatus, and hence the access to the tympanic mem- 

 brane, of sounds travelling in a particular direction,- this also 

 assists our judgment of the direction of sounds. Hence, by 

 tying back the ears and affixing artificial ears, differing in shape 

 or position from the natural ones, we may make false judgments 

 in this matter. 



Moreover, in forming a judgment as to the direction of 

 sounds we appear to be guided by something more than the 

 mere relative intensity of the sounds falling on the two ears. 

 When a complex sound emanates from a body on one side of us, 

 the constituent vibrations do not travel equally and uniformly 

 over and around the head ; some are refracted more than others, 

 so that they do not reach the two ears equally ; and besides 

 when they reach them are not equally reflected by the two 

 pinnae. In this way partial tones of different pitch, and this 

 applies especially to high tones, reach the two tympanic mem- 

 branes in unequal intensities, and the sound of which they form 

 part appears as heard by the one ear of a quality slightly differ- 

 ent from that heard by the other ear ; this difference of quality, 

 like the difference in mere intensity of the sound as a whole, 

 serves as a basis for recognizing the direction of the sound. 

 Such a difference will be more marked in the complex sounds 

 which we call noises than in purer and more simple musical 

 sounds ; and, as a matter of fact, our appreciation of direction 

 is more accurate in the case of noises than of musical sounds. 

 An exception to this rule is met with in the case of the human 

 voice, the direction of which, though it is as a whole a musical 

 sound, can be judged better than even that of a noise ; but 

 noises enter largely into the human voice, and besides we are 

 much more practised in relation to it than in relation to any 

 other kind of sound. All our judgments of the direction of 

 sounds are however at the best imperfect. 



§ 635. Our judgment of the distance of sounds is even still 

 more limited. A sound whose characters we know appears to 

 us near when it is loud, and far off when it is faint. A blind- 

 fold person will be unable to distinguish between the difference 



