396 ELEMENTARY PHYSIOLOGY less. 



Let us now revert to the relation of the semicircular 

 canals to giddiness. If a glass containing water in which 

 some powder has been placed be rotated, the observer will 

 see, by looking at the powder, that whilst the glass rotates 

 the water does not. This is so at first at all events, but if 

 the rotation be maintained the water begins to rotate 

 also, and lastly, if the glass be suddenly stopped the 

 water will continue to rotate. Suppose further that there 

 was a projection from the side of the glass, pointing 

 towards the centre : when the rotation of the glass com- 

 menced, this projection would have to go through the 

 stationary water ; there would therefore be an increased 

 pressure of fluid on the side of the projection which was 

 breasting the water and a decreased pressure on the side 

 which was retreating from it ; tliis would be so until the 

 water acquired a rotation as rapid as that of the glass. 

 When the glass was stopped the water would then stream 

 on past the projection and would press on the opposite 

 side to that whicli had at first suffered the increased 

 pressure ; that is to say the side which would have 

 had to breast the stream if the glass had originally been 

 rotated in the opposite direction. If in our minds we 

 replace the glass of water by the horizontal semicircular 

 canal, and the projection by the crista in the ampulla, and 

 if we suppose further that the hair-cells on the crista can 

 appi'eciate the alterations in the pressure of fluid upon 

 them, we will see, firstly, that they would at once acfjuaint 

 us of any rotation of the body. Were the body to rotate 

 in the direction of the clock the endolymph would press 

 on one side of the crista ; were it to rotate in the reverse 

 direction the endolymph would press on the ojjposite 

 side ; were it to rotate clockwise long enough for the endo- 

 lymph to acquire the rotation of the body, and then sud- 

 denly to stop, the pressure of fluid on the hair-cells of the 

 crista would be similar to that which would be occasioned 

 by the commencement of a rotation against the hands of 

 the clock, and the person would be deceived into thinking 



