Implications of the Theories of Nerve Action 189 



sponse as the clasping act by the arms of the decerebrated 

 male, frog, and substituting the term tropism. "It is better,'' 

 he says, "to call them tropisms since the organism as a whole 

 is involved." 3 



But Loeb's most illuminating recognition of the organis- 

 mal character of tropisms is found not in any positive 

 acknowledgements but in his statements of what a tropism 

 consists of essentially. This he has told us many times and 

 in varied phraseology during the last twenty-five years. The 

 following from The Organism as a Whole will serve well as 

 the basis of our consideration : "Animals possess photo- 

 sensitive elements on the surface of their bodies, in the eyes, 

 or occasionally also in epithelial cells of their skin. These 

 photosensitive elements are arranged symmetrically in the 

 body and through nerves are connected with symmetrical 

 groups of muscles. The light causes chemical changes in 

 the eyes (or the photosensitive elements of the skin). The 

 mass of photochemical reaction products formed in the 

 retina (or its homologues) influences the central nervous sys- 

 tem and through this the tension or energy production of 

 the muscles. If the rate of photochemical reaction is equal 

 in both eyes this effect on the symmetrical muscles is equal, 

 and the muscles of both sides of the body work with equal 

 energy; as a consequence the animal will not be deviated 

 from the direction in which it was moving. This happens 

 when the axis or plane of symmetry of the animal goes 

 through the source of light, provided only one source of 

 light be present." 4 



Reduced to its lowest terms, the tropistic mechanism de- 

 scribed here and in many other places consists of: first, 

 portions of the body surface having specific irritability, as 

 receptor organs, these being usually paired and usually 

 symmetrical, though often unsymmetrical ; second, muscles 

 as effectors, almost always paired and usually symmetrical; 

 third, nerves, variously constituted, as conducting paths 



