238 THE HUMAN BODY 



in relation to the walls of the canals may act as stimuli by caus- 

 ing a swaying of the projecting hairs of the ampullae (Fig. 75). 

 Place a few small bits of cork in a tumbler of water, and rotate 

 the tumbler; at first the water does not move with it; then it be- 

 gins to go in the same direction, but more slowly; and, finally, 

 moves at the same angular velocity as the tumbler. Then stop 

 the tumbler, and the water will go on rotating for some time. 

 Now if the head be turned or rotated in a horizontal plane simi- 

 lar phenomena will occur in the endolymph of the horizontal 

 canal; if it be bent sidewise in the vertical plane, in the ante- 

 rior vertical canal; and if nodded, in the posterior vertical; the 

 hairs moving with the canal would meet the more stationary 

 water and be pushed and so, possibly, excite the nerves at the 

 deep ends of the cells which bear them, and generate afferent 

 impulses which will cause the general nerve-centers of bodily 

 equilibration to be differently acted upon in each case. Under 

 ordinary circumstances the results of these impulses do not be- 

 come prominent in consciousness as definite sensations; but they 

 are probably always present. If one spins round for a time, the 

 endolymph takes up the movement of the canals, as the water in 

 the tumbler does that of the glass; on stopping, the liquid still 

 goes on moving and stimulates the hairs which are now stationary; 

 and we feel giddy, from the ears telling us we are rotating and the 

 eyes that we are not; hence difficulty in standing erect or walking 

 straight. A common trick illustrates this very well: make a per- 

 son place his forehead on the handle of an umbrella, the other 

 end of which is on the floor, and then walk three or four times 

 round it, rise, and try to go out of a door; he will nearly always 

 fail, being unable to combine his muscles properly on account of 

 the conflicting afferent impulses. This and the feeling of rotation 

 in the contrary direction when a previous rotation ceases become 

 readily intelligible if we suppose feelings to be excited by relative 

 movements of the endolymph and the canals inclosing it. 



The sense of equilibrium as mediated by the semicircular canals 

 is a dynamic sense, one dealing with equilibrium of motion. That 

 we have also a static sense of equilibrium, which tells us our posi- 

 tion when at rest is well known. The swimmer immersed in 

 water knows perfectly whether he is on his face or on his back; 

 whether his head is up or down. This static equilibrium sense is 



