SYMMETRY RELATIONS 21 



changes in tension will be produced in the symmetrical 

 muscles on both sides of the body with which the active 

 brain elements are connected. a On account of the sym- 

 metrical character of all the changes no deviation from 

 the original direction of motion will occur. If, however, s ~ 

 one eye is illuminated more than the other eye, the influ- 

 ence upon the tension of symmetrical muscles will no 

 longer be the same and the animal will be forced to deviate 

 from the original direction of motion. 



We have thus far considered only the relation between 

 right and left. Aside from this symmetry relation we il 

 have polarity relations, between apex or head and base ' 

 or tail end. Just as we found that the morphological 

 plane of symmetry is also a dynamical plane of symmetry, 

 we find that with the morphological polarity head-tail is 

 connected a dynamic polarity of motion, namely, forward 

 and backward. This will become clear in the next chapter 

 on forced movements. 



Physiologists have long been in the habit of studying 

 not the reactions of the whole organism but the reactions 

 of isolated segments (the so-called reflexes). While it 

 may seem justifiable to construct the reactions of the 



a Physiologists assume that stimulations are constantly sent from the 

 brain to the muscles and that this maintains their tension, v. Uexkiill 

 uses the term that "tonus " is sent out to the muscle and that the brain 

 is a reservoir of " tonus " as if the latter were a liquid. The writer wonders 

 whether it might not be wiser to substitute for such metaphors hypotheses 

 in terms of chemical mass action. -'Constant illumination causes a sta- 

 tionary process in photosensitive elements of our eye, in which the mass of 

 the reaction product is determined by the Bunsen-Roscoe law. We assume, 

 moreover, that in proportion to this photochemical mass action correspond- 

 ing chemical reactions take place in the brain elements with which the eyes 

 are connected; and that as a consequence corresponding chemical reactions 

 take place in the muscles by which the tension of the latter is determined. 

 These processes in the muscles may possibly consist in the establishment of 

 a definite hydrogen ion concentration. Such hypotheses! have the advantage 

 over the " stimulation " hypothesis that they can be tested by physico- 

 chemical measurements. 



