M 



SENSE OP HEARING PERCEPTION OF DIRECTION, DISTANCE. 795 



tube ; the intensity of external sounds, and also that of the individual's own 

 voice, as heard by himself, is then much increased. 



649. Many facts prove, however, that the fluid of the Labyrinth may be 

 thrown into vibration in other ways than by the Tympanic apparatus. Thus 

 in Osseous Fishes, it is only by the vibrations transmitted through the bones 

 of the head, that hearing can take place. There are many persons, again, 

 who can distinctly hear sounds which are thus transmitted to them ; although, 

 through some imperfection of the tympanic apparatus, they are almost in- 

 sensible to those which they receive in the ordinary way. It is evident, 

 where this is the case, that the nerve must be in a state fully capable of func- 

 tional activity; and on the other hand, where sounds cannot thus be perceived, 

 there will be good reason to believe that the nerve is diseased. Urbautschitsch 1 

 finds that there are three deaf-spots in each ear, or spots in which a vibrating 

 tuning-fork passed in front of the ear is not heard. The two chief areas have 

 the form of two little triangles, one of which extends forwards and upwards, 

 the other backwards and upwards. 



650. The power of distinguishing the direction of sounds appears to be, in 

 Man at least, for the most part acquired by habit ; for it is some time before 

 the infant seems to know anything of the direction of noises which attract 

 his attention. Our judgment as to this point is probably assisted, in most 

 cases, by a difference in the intensity of the sensations received through the 

 two ears respectively ; but since we have a certain power of appreciating di- 

 rection when one ear alone is used, this power must depend upon an exercise 

 of perceptive discrimination (which is probably acquired, rather than in- 

 tuitive) in regard to the impressions which we receive through its means; 

 and it has been already mentioned that the Semicircular canals ( 647) ap- 

 pear to furnish the instrumentality by which our minds are enabled to take 

 cognizance of such differences. The idea of the distance of the sonorous 

 body is another acquired perception, depending principally upon the loud- 

 ness or faintness of the sound, when we have no other indications to guide 

 us. In this respect there is a great similarity between the perception of the 

 distance of an object, through the Eye by its size, and through the Ear by 

 the intensity of its sound. When we are acquainted with the usual intensity 

 of its sound, we can judge of its distance ; and vice versa, when we know its 

 distance, we can at once form an idea of its real strength of tone from that 

 with which our ears are impressed. In this manner, the mind may be 

 affected with corresponding deceptions through both senses ; for as, in the 

 Phantasmagoria, the figure being gradually diminished while its distance 

 remains the same, it appears to the spectators to recede (the illusion being 

 more complete if its brightness be at the same time diminished) ; so the 

 effect of a distant full military band gradually approaching, may be alike 

 given by a corresponding crescendo of concealed instruments. It is upon the 

 complete imitation of the conditions which govern our ideas of the intensity 

 and direction, as well as of the character, of sounds, that the deceptions of 

 the Ventriloquist are founded. A very curious instance of the degree in 

 which our auditory interpretation is affected by other sense-perceptions, is 

 afforded by the "Ghost" exhibition which has recently been so popular; 

 for it is scarcely possible to avoid referring to the place where they seem 

 to be made, sounds which are really produced elsewhere. 



651. The Auditory sense, like the visual, may vary considerably among 

 different individuals, both as regards its general acuteness, and as respects 

 its discriminative power for particular classes of impressions. Much de- 

 pends upon the habit of attention to its indications ; and thus it comes to 



1 Centralblatt f. d. Med. Wiss., 1872, No. 3. 



