1 82 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



to respond each to a special rate of vibration, somewhat 

 as a note on a violin falling on all the strings of a piano 

 only one will respond, viz., that one which vibrates in 

 unison — co-vibrates — with the given note. 



The semicircular canals with their ampullae and 

 terminal hairs may concur with the vestibular sac and 

 otoliths in perceiving sound as noise, but they are also 

 supposed to have another function. 



We have not yet drawn attention to the remarkable 

 fact that these three canals are set in three rectangular 

 plains, one vertical fore and aft, one vertical from side 

 to side, and the third horizontal (Fig. 116, page 183). 

 The least movement of the head back and forth, as in 

 nodding, would move the water in the vertical fore-and- 

 aft canal and would be perceived by the hairs of its 

 ampulla. Similarly movements of the head from side to 

 side, as in wagging, would be perceived by the ampulla 

 of the canal set in the vertical transverse plain ; while 

 rotation of the head on the spinal column, as in the sign 

 of negation, would be perceived by the ampulla of the 

 horizontal canal. In other words, these canals with 

 their ampullae are a most delicate indicator of the 

 position and movement of the head, and therefore 

 necessary for maintaining the equilibrium of the body. 

 Surely this is a fundamental and most important func- 

 tion. Many experiments seem to substantiate this 

 view.* 



The perception of direction, which is so mathematically 

 exact in the case of the eye, is extremely inexact in the 

 case of the ear. It is this inexactness which is utilized 

 by the ventriloquist in producing his deceptions. 



* Some would go further and say this is the sole function, and 

 others still further and say the only organ of hearing is the 

 cochlea. This, however, seems improbable. 



