THE NERVOUS SYSTEM 335 



developed into ears ; and the ear was in its earliest phases, and 

 still is, something more than an organ of hearing. Its semi- 

 circular canals give information of displacements in space. 

 Knowledge of the position of its body is, to a fish, of far more 

 importance than its ability to hear breakers on the rocks. Three 

 looped tunnels, opening at either end into a common chamber, 

 are hollowed in the bone which contains the ear (cf. Fig. 38). 

 Placed at right angles one to the other, they occupy all three 

 dimensions of space. Open a notebook until, one of its covers 

 lying horizontally, the other is vertical, and place a sheet of 

 paper vertically against the bottom of the pages. A curved 

 line drawn on each of these three surfaces will represent the 

 three semicircular canals. Arrange another notebook in the 

 same way, and let the two rest on the table with the two 

 vertical covers inclining one to the other, anteriorly, at an angle 

 of 90 degrees. The six surfaces will be in the planes of the 

 six semicircular canals. Within each bony canal is a mem- 

 branous tube, to which nerves are distributed, filled with 

 fluid. When the position of the head is changed, the fluid 

 within the membranous tubes slides on their walls. It is left 

 behind at the moment the movement commences. It over- 

 takes its receptacle when the movement stops. The stimulus 

 received by the nerve-endings is recognized as indicating an 

 alteration in the orientation of the head. If the movement of 

 the fluid is violent, as when one waltzes, the loss of the sense 

 of position disconcerts the brain to such an extent that giddi- 

 ness results. For a time the quiet assurance upon which so 

 much depends, that one knows how the body stands in relation 

 to its surroundings, gives way to a chaos of sensations. From 

 the nature of the case, the information which the semicircular 

 canals afford relates to change. They give no help in ascer- 

 taining the position of the head when it is at rest. This must 

 be the reason, although the connection is not very clear, for 

 the waning of the effect in consciousness when stimulation is 

 prolonged, and also for the very marked after-sensation. At 

 the commencement of a voyage attention may be unpleasantly 

 attracted to the rolling of the ship. After a few days it ceases 

 to be noticeable ; yet when the voyager, the night after landing, 

 wakes in the dark, he finds his bed-room as unsteady as his 

 cabin. Rising hurriedly, the attempt to adjust his position to 



