20 TROPISMS 



the photochemical reaction products produced in the same 

 time will be equal in both eyes. What is true for the eyes 

 is true for all symmetrical elements of the surface of an 

 animal. 



The median plane is also the plane of symmetry for the 

 muscles and the muscular activity of the body. Sym- 

 metrical muscles possess under equal conditions equal 

 tension and symmetrical muscles are antagonistic to each 

 other in regard to moving the body to the right or left. 



We say that impulses go from the central nervous 

 system to the muscles ; and from the surface of the body 

 to the central nervous system. According to our present 

 knowledge that which is called a nervous impulse seems 

 to consist of a wave of chemical reaction traveling along 

 a nerve fiber. The central nervous system is also sym- 

 metrical and, moreover, we may conceive a projection of 

 the elements of the surface of the body upon the ganglion 

 cells and from here to the muscular system of the body. 

 The complications in this system of projections consti- 

 tute the difficulties in our understanding of the structure 

 of the brain. This idea of a projection of the sense organs 

 or the surface of the body upon the brain will explain 

 why the morphological plane of symmetry of an organism 

 is also its plane of symmetry in a dynamical sense. When 

 symmetrical elements of the eyes are struck by light of 

 the same wave length and intensity, the velocity of photo- 

 chemical reactions will be the same in both eyes. Sym- 

 metrical spots of the retina are connected with symmetri- 

 cal elements in the brain and these in turn are connected 

 with symmetrical muscles. As a consequence of the equal 

 photochemical reactions in the symmetrical spots of the 

 retina equal changes are produced in the symmetrical 

 brain cells with which they are connected, and equal 



