ORIENTATION IN SPACE 359 



influence of the eyes is removed, the patient finds difficulty in 

 standing and in walking (Romberg's sign). 



III. Chemotropism plays an important part in the life of the 

 lower organisms. By it, the animal is drawn towards or draws 

 away from certain chemical substances. The organ stimulated 

 asymmetrically is oriented so that the stimulating impacts on it 

 are symmetrical. 



IV. Orientation in space is determined mainly by three factors, 

 light, tactile sense and gravitation. Normal equilibrium or normal 

 geotropic orientation is defined as that position in which the plane 

 of symmetry of the animal passes through the centre of the earth. 

 Any deviation from that position causes unilateral stimulation 

 and corrective movements are instituted. The tight-rope walker 

 perceives that his centre of gravity is tending towards unstable 

 equilibrium and voluntary (though generally sub-consciously) 

 corrects his balance. In the labyrinths, we have a delicate 

 mechanism for detecting alterations in our orientation in space. 



Sufficient has been said to show the nature and indicate the 

 mechanism of those actions termed tropisms. In principle they 

 depend on unilateral stimulation of a symmetrical animal. How 

 far they can be accepted as explanations of all the instinctive 

 actions of the lower organisms or of any of the actions of the 

 higher animals remains an open and debatable question. 



