The Senses of Animals. 271 



for the fluid within the sac to flow into the ampulla, and 

 for the fluid around the semicircular canal to flow into the 

 cavity in which the sac lies. These movements will con- 

 spire to stretch the membranous ampulla, and thus to 

 stimulate the hair-cells. This stretching will not take 

 place in that canal if the rotation be in the reverse direction. 

 But on the opposite side of the head is another canal in 

 the same plane, but turned the other way. In the reversed 

 rotation the ampulla in this canal will lead, and its hair- 

 cells will be stimulated. Thus by means of the two canals 

 on either side of the head in the same plane, rotation in 

 either direction can be appreciated. And since there are 

 two other pairs of semicircular canals in two other planes, 

 rotation in any direction will be recognized by means of 

 one or more of the six canals. 



It is thus by means of the semicircular canals that we 

 can appreciate acceleration of rotatory motion.* But we 

 can also appreciate acceleration of movements of translation 

 — forwards or backwards, up or down. And Professor 

 Mach has suggested that it is through the stimulation of 

 the hair-cells in the patch in the sac itself (the so-called 

 macula acustica) that we are able to appreciate these changes. 

 The otoliths, held loosely and lightly in position by the 

 gelatinous substance in which they are embedded, may, 

 through their inertia, aid in the stimulation of the sense- 

 hairs. 



And this naturally suggests the question whether those 

 sense-organs in the invertebrates which contain otoliths 

 may not be regarded with more probability as organs for 

 the appreciation of changes of motion than as auditory 

 organs. This for some years has been my own belief. I 

 have always felt a difficulty in understanding how the 

 otoliths are set a-dance by auditory vibrations. But their 

 inertia would materially aid in the appreciation of changes 

 of motion. In some forms the otoliths are held in suspen- 

 sion in a gelatinous material. In others — the molluscs, 



* It is interesting to note that in the blind-fish (Amhlyopsis spelxus) the 

 semicircular canals are, accordiug to Wyman, unusually large. 



