1022 AUDITORY PERCEPTIONS. [BOOK in. 



pushed well up into the auditory passages, the sounds heard 

 through the instrument seem to come from the roof of the ob- 

 server's own mouth. 



The difference between such an abnormal mode of hearing 

 and ordinary hearing does not lie in the fact that in the former 

 case the tympanic membrane is not used at all ; for even when 

 the external passage is filled with fluid, a layer of air which 

 always adheres to the tympanic membrane permits at least a 

 certain amount of vibration of that membrane ; and on the other 

 hand when the sound is actually generated in the roof of the 

 mouth, and rightly judged to be generated there, the tympanic 

 membrane by its vibrations conducts the greater part of the 

 sound to the internal ear. How it is that the passage of the 

 vibrations through the external passage imparts to the sensation 

 this attribute of outwardness is not clear. Indeed certain 

 sounds may be made to lose this particular outwardness, though 

 the external passage be still employed. If two musical sounds 

 of the same pitch be listened to with the two ears separately 

 by means of two telephones, the sound will, under certain 

 conditions, appear to originate somewhere in the head of the 

 observer. 



634. In the second place our appreciation of the particular 

 quarter from which a sound, recognized by help of the external 

 passage to be of outward origin, has travelled is dependent on 

 our using two ears. As our ordinary vision is largely binocular, 

 so our ordinary hearing is, to a still larger extent, binaural. In 

 the case of the ear there are no sharp limitations to the range of 

 the organ of either side; through the medium of the air and 

 external auditory passage or of the hard parts of the head a 

 sound which affects one ear affects to a certain extent the other 

 ear also ; hence all our hearing is, under ordinary circumstances, 

 binaural. And in some such way as two visual sensations 

 excited in " corresponding parts " of the two retinas are fused 

 into one, so every sound which reaches us is heard not as two 

 sounds, one by one ear and the other by the other, but as one 

 sound by the two ears together. 



When the sounding body is on one side of the head, say the 

 right side, the sensations excited through the right internal ear 

 are more powerful than those excited through the left internal 

 ear ; we are not distinctly conscious of the difference between 

 the two sensations, the combined effect is a single sensation; 

 but the difference does affect our consciousness in a certain 

 way, and that affection of consciousness serves as a basis for the 

 judgment that the sounding body is somewhere on our right 

 hand. Hence we are able to judge the lateral much more 

 readily than the fore and aft position of a sounding body. If a 

 tuning-fork be held in the median vertical plane over the head, 

 the eyes being shut, though it is easy to recognize it as being in 



