CHAP, in.] HEARING, SMELL, AND TASTE. 565 



Auditory Judgments. 



In seeking for the cause of our visual sensations we invariably 

 refer to the external world. The sensation caused by a direct 

 stimulation of the optic nerve or retina by a blow or a galvanic 

 current, we identify with that caused by a flash of light. A sensa- 

 tion arising from any stimulation of the left side of our retina we 

 regard as caused by some object on the right-hand side of our ex- 

 ternal visible world. In a similar way, but to a less extent, we 

 project our auditory sensations into the world outside us, and when 

 the auditory nerve is affected we seek the cause in vibrations start- 

 ing at a greater or less distance from us. We do not think of the 

 sound as originating in the ear itself. 



This mental projection of the sound is much more complete 

 when the ear is stimulated by vibrations reaching it through the 

 membrana tympani than when the vibrations are conducted by the 

 solids of the head directly to the perilymph of the labyrinth. 

 When the meatus externus is filled with fluid and the vibrations 

 of the membrana tympani are in consequence interfered with, the 

 apparent outwardness of sounds is to a very large extent lost; 

 sounds, however caused, seem under these circumstances to arise in 

 the ear. Hence it would seem that the vibrations of the mem- 

 brana tympani, or possibly the action of the muscles attached 

 to the ossicula, give rise to obscure sensations of which, by them- 

 selves, we are not distinctly conscious, but which nevertheless lead 

 us to judge that the sounds heard by means of the tympanum 

 come from outside the ear. 



Our judgment of the distance of sounds is very limited. A 

 sound whose characters we know appears to us near when it is loud, 

 and far off when it is faint. A blindfold person will be unable to 

 distinguish between the difference of intensity produced on the 

 one hand by a timing-fork being held before him, first with the 

 broad edge of the fork toward him and then with the narrow edge, 

 and the difference on the other hand caused by the removal of the 

 tuning-fork to a distance. We can on the whole better appreciate 

 the distance of noises than of musical sounds. 



Our judgment of the direction of sounds is also very limited. 

 Our chief aid in this is the position in which we have to place the 

 head in order that we may hear the sound to the best advantage. 

 If a tuning-fork be held in the median vertical plane over the 

 head, though it is easy to recognise it as being in the median plane, 

 it becomes very difficult when the eyes are shut to say what is its 

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



