i8 THEORETICAL BIOLOGY 



would mean the removal of this canvas and would deprive 

 movements of their fixed position in space. 



In order to imagine the injury you would suffer from 

 such a loss, shut your eyes, and with your hand quickly bring 

 a fiat object, such as a book, from a distance to the boundary 

 between right and left, then to the boundary between above 

 and below, and finally to that between before and behind. 

 You can then convince yourself of the exactness with which 

 the bounding planes are found, and where they intersect. We 

 carry around with us near the tip of the nose an invisible co- 

 ordinating organ as the basis of our movements, which is 

 quite unaffected by all the influences of the outside world 

 and by the position of the rest of the body. The loss of this 

 basis would undoubtedly have the most injurious effect on 

 the certainty of our movements. 



The planes of direction influence most of all the move- 

 ments of our eyes ; their loss would therefore be felt there 

 most. We determine the position of the various objects in 

 space by extremely rapid movements of the eyes ; these 

 movements start from the middle position, and go to and fro. 

 It is easiest to verify the continually repeated return of the 

 eye to the middle position by watching a person who is looking 

 at rapidly passing objects. The to-and-fro movement of his 

 eyes is known as normal nystagmus. Very different is the 

 abnormal nystagmus which we observe in animals that have 

 lost their semicircular canals : here the eyes roll continually 

 to and fro from one corner of the orbit to the other, as though 

 they were seeking for the lost median position. Animals in 

 that state prefer to remain in the dark, and only very gradually 

 do they re-acquire their normal eye-movements. 



The picture they present corresponds in all details with 

 the condition we should be in if we were suddenly deprived of 

 the direction-planes. It is only through knowing the planes 

 of direction that we are always able to bring our eyes back 



