CHAP, iv.] HEARING. 1381 



binaural stethoscope be pushed well up into the auditory passages, 

 the sounds heard through the instrument seem to come from the 

 roof of the observer'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 rilled 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 out- 

 wardness 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. 



855. 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 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 



882 



