HEARING. 805 



fall of the sound, known as " beats." When the beats follow each other as 

 rapidly as 132 in a second they cease to be recognized, that is to ay, the 

 sensations which they cause become fused. Before they disappear they give 

 a peculiar disagreeable roughness to the sound. The pleasure given by 

 musical sounds depends largely on the absence of this incomplete fusion of 

 sensations. 



720. Corresponding to entoptic phenomena there are various entotic 

 phenomena, sensations or modifications of sensations originating in the 

 tympanum or in the labyrinth ; moreover, sensations of sound may rise in the 

 auditory nerve or in the brain itself, without any vibration whatever falling 

 on the labyrinth. 



Auditory Judgments. 



721. In seeking for the cause of our visual sensations we invariably 

 refer to the external world. The sensation caused by 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 sensation 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 external 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 starting 

 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 membrana tympani, or pos- 

 sibly the action of the muscles attached to the ossicula, give rise to obscure 

 sensations of which, by themselves, we are not distinctly conscious, but which 

 nevertheless lead us to judge that the sounds heard by means of the tym- 

 panum come from outside the ear. 



722. 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 tuning-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. 



723. 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 rec- 

 ognize 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 

 toward the front or back of the head. In this respect, too, our appreciation 

 is more accurate in the case of noises than of musical sounds, with the excep- 

 tion of those given out by the human voice, the direction of which can be 

 judged better than even that of a noise. 



