THE EAR 



367 



third in a vertical plane, in a front-to-back direction. Thus, 

 one of these canals is located in each of the three planes of 

 space, in each of the maia directions in which the body can be 

 inclined. 



If the location and structure of the canals is clear, their 

 manner of functioning can be readily understood. They are 

 filled with endolymph, and when the head is rotated, this 

 fluid acts as water does in a pail or glass jar when the latter 

 is turned around in 



the hands. The liquid ,Lime granules 



scarcely moves, but ^A^iS3fi^b.,.. tfafrCelh 

 the pail or jar slips 

 around outside the 

 liquid. If there were 

 threads attached to 

 the sides of the pail, 

 these would be bent 

 back and forth by the 

 water slipping over 

 them as the pail was 

 turned first one way 

 and then the other; in 



like manner, if the head is tipped from side to side, or forward 

 and backward, the endolymph in the canals tends to stay in its 

 original position, while the canals slip around it. This makes 

 the hair-like projections on the crests in the ampulke bend to 

 and fro, and since nerve fibres end in these ampullae, this sway- 

 ing of the threads creates a message which is carried to the 

 brain. In tipping in any direction, the lime granules will set- 

 tle on different parts of the ampullae. This will irritate 

 nerves ending in these regions, and the result is that one has 

 a sensation of inclining from side to side, of falling or of rolling 

 over, as the case may be. 



Of course all the movements one makes are not in the exact 

 planes in which the semicircular canals lie, but they are at 



FIG. 183. A SECTION OF ONE OF THE AMPULLAE 



OF THE SEMI-CIRCULAR CANALS (Kolliker) 



