JUDGMENT OF DISTANCE, ETC., BY SOUND. 203 



the sound appear to proceed from a near or a distant object, he irre- 

 sistibly leads us into acoustic error. 



Education or experience is required to enable us to appreciate dis- 

 tances accurately by this sense; as well as to judge of their position. 

 In the case, detailed by M. Magendie, 1 of a boy, who, after having been 

 entirely deaf until the age of nine, was restored to hearing by M. 

 Deleau, by means of injections thrown into the cavity of the tympanum 

 through the pharyngeal extremity of the Eustachian tube, one of the 

 most remarkable points was his difficulty in acquiring a knowledge of 

 the position of sonorous bodies. In forming an accurate judgment on 

 this subject we seem to require the use of both ears. In all other cases 

 an impression made upon one only would perhaps be sufficient. The 

 common opinion is, that to judge of the direction of a sound we com- 

 pare the intensity of the impression on each ear, and form our deduc- 

 tions accordingly; and that if we close one ear we are led into errors, 

 which are speedily dissipated by employing both. Still we are often 

 deceived even under these last circumstances, and are compelled to call 

 in the aid of sight. The blind afford us striking examples of accuracy, 

 in their perceptions by the ear. In the Belisar of Zeune, the case of a 

 blind man is cited from Diderot; who, guided by the direction of the 

 voice, struck his brother in a quarrel on the forehead, with a missile, 

 which brought him to the ground. 2 



Mr. Wheatstone supposes, that the perception we have of the direc- 

 tion of sounds arises solely from the portion transmitted through the 

 solid parts of the head, which, by affecting the three semicircular 

 canals, situate in planes at right angles with each other, with different 

 degrees of intensity, according to the direction in which the sound is 

 transmitted, suggests to the mind the corresponding direction. If the 

 sound be transmitted in the plane of either of the semicircular canals, 

 the nervous matter in that canal will be more strongly acted on than 

 that in either of the other two; and if in any plane intermediate be- 

 tween two of the rectangular planes, the relative .intensities in these 

 two canals corresponding therewith will vary with the direction of the 

 intermediate plane; 3 and it has been regarded by Dr. Carpenter 4 as a 

 powerful argument in support of this view, that in almost every instance 

 in which these canals exist at all, they hold the same relative position 

 to each other as in man; their three planes being nearly at right angles 

 to one another. He properly, however, adds, that the idea must be 

 regarded as a mere speculation, the value of which cannot be decided 

 without an increased knowledge of the laws according to which sonorous 

 vibrations are transmitted. 



If these vibrations, before reaching the ear, be deflected from their 

 course, we are liable to deception, mistaking the echo for the direct or 

 radiant sound. 



The ideas of magnitude acquired by the ear are few, and to a trifling 

 extent only. They occasionally enable the blind to judge of the size 



1 Journal de Physiologie, v. 223. 



2 Rudolphi, Grundriss der Physiologie, s. 149, Berlin, 1823. 



3 Journal of Science, New Series, ii. 67, London, 1827. 



4 Human Physiology, 359, London, 1842. 



