510 THE SPECIAL SENSES. 



not well understood. It has been thought to be the office of the 

 semicircular canals to determine the direction from which the sono- 

 rous impulses are propagated. This opinion was based upon the 

 curious fact that these canals, always three in number, are placed 

 in such positions as to correspond with the three different directions 

 of vertical height, lateral extension, and longitudinal extension; 

 for one of them is nearly vertical and transverse, another vertical 

 and longitudinal, and the third horizontal in position. The sono- 

 rous impulses, therefore, coming in either of these directions, would 

 be received by only one of the semicircular canals (by direct con- 

 duction through the bones of the head) perpendicularly to its own 

 plane; and an intermediate direction, it was thought, might be 

 appreciated by the combined effect of the impulse upon two adja- 

 cent canals. 



Enough has already been said, however, in regard to the com- 

 munication of sound directly through the bones of the head to the 

 internal ear, to show that this cannot be the way in which the direc- 

 tion of sound is ascertained. Indeed,, when we hear any loud and 

 well-marked sound coming from a particular region, such as the 

 music of a military band or the whistle of a locomotive, we have only 

 to close the external ears to lose our perception both of the sound 

 and its direction. The direction of sonorous impressions is appre- 

 ciated in a different way. In the first place, we feel that the sound 

 comes from one side or the other, by its making a more distinct 

 impression on one ear than the opposite; and by inclining the 

 head slightly in various directions, we easily ascertain whether the 

 sound becomes more or less acute, and so judge of its actual source. 

 Many of the lower animals, whose ears are very large and movable, 

 use this method to great extent. A horse, for example, when upon 

 the road, often keeps his ears in constant motion, feeling, as it were, 

 in the distance, for the origin of the various sounds which excite 

 his attention. 



Beside the above, we are further assisted in our judgment of the 

 direction of sounds by our previous knowledge of the localities, 

 the direction of the wind, and the manner in which the sound is 

 reflected by surrounding objects. When these sources of informa- 

 tion fail us, we are often at a loss. It is notoriously difficult, for 

 example, to judge of the place of the chirping of a cricket in a 

 perfectly closed room, or of the direction of a bell heard on the 

 water in a thick fog. 



The sense of hearing has a much closer analogy with ordinary 



