302 COMPARATIVE PHYSIOLOGY OF THE BRAIN 



main axes of our body : (i) innervations to move our 

 eyes from right to left, or vice versa ; (2) innervations 

 to move them up and down ; and (3) move them 

 from a near to a far object, or vice versa (8). The 

 first motion takes place along the transverse axis, the 

 second along the longitudinal, and the third along 

 the dorso-ventral axis. The experiments on the hori- 

 zontal semicircular canals of the ear show that the 

 stimulation of this canal produces motions of the eyes, 

 head, or even of the whole animal in the plane of this 

 canal. Our experiments on galvanotropism indicate 

 the existence of a simple relation between the orien- 

 tation of certain motor elements in the central nerv- 

 ous system and the direction of the motions produced 

 by their activity. This is supported to a certain 

 extent by the experiments on the crura cerebelli. It 

 thus seems possible that simple geometrical relations 

 of structure are responsible for the fact that all our 

 innervations may be reduced to three classes deter- 

 mined by the main axes of our body. On the other 

 hand, Mach furnished the proof that the will or the 

 process of innervation for a motion is of the same 

 character as the process of space-sensations (5, 6). 

 The will to move our eyes to a certain point and 

 the space-sensation itself can be added algebraically. 

 The experiences derived from space illusions caused 

 by imperfect motility of the eyes or hands agree with 

 this view. Moreover, Mach has proved that we recog- 

 nise the geometrical symmetry of two figures very 

 easily only when the axis of symmetry coincides with 



