WALTER E. GARREY 103 



maintained and is proportional to the difference in illumination of 

 the two sides. The tonic changes produced under these conditions 

 are entirely analogous to the changes in muscle tonus produced under 

 the influence of a gradually increasing galvanic current and are in 

 marked contrast to the twitch with which a muscle responds to a 

 sudden stimulus. The analogies between electrotonus and what we 

 may now call phototonus, have been considered in some detail in a 

 previous communication.^ 



Lyon found^ that all compensatory motions of insects were abohshed 

 by blackening their eyes, while the positions of the eyes of Crustacea 

 were so altered that he concluded that "light may cause .... an 

 unequal tension of associated muscles." Ewald^ confirmed this 

 in a quantitative way for the orientation of the eye of Daphnia. 

 Holmes'^" (1908) also describes an "increase in the tension of the leg 

 muscles (and eye stalk of fiddler crabs) brought about by strong illumi- 

 nation." Delage/"^ in 1887, had already concluded that the eyes of 

 insects seem to be their most important organs of equilibrium, and 

 Radp2 believed them to be true organs of muscle tonus. Even ver- 

 tebrates show that illumination of the eyes has a distinct effect upon 

 the tonus of associated muscle groups as was demonstrated by Gar- 

 rey,^^ and by Lyon's proof that the rheotropic orientation of several 

 species of fish depended upon the optical effects of a "relative motion 

 between the fish and its solid surroundings."^* Loeb's^^ experiments 

 on the compensatory motions of horned toads show a similar influence 

 of light upon the tonus of associated muscle groups, since the effects 

 of light and rotation, upon compensatory motions, were summed alge- 

 braically. In man, the eyes also have an effect, upon the tonus of 

 the body muscles, subordinate only to that of the internal ear and the 

 muscle sense. This is illustrated in the so called visual nystagmus of 

 the eyes. It may prove true that the relation of the eyes to Rom- 



8 Lyon, E. P., Am. J. Physiol., 1900, iii, 86. 

 ' Ewald, W. F., Science, 1913, xxxviii, 236. 

 i" Holmes, S. J., /. Comp. Neurol, and Psychol., 1908, xviii, 493. 

 ^^ Delage, Y., Arch. zool. exp. el gen., 1887, series 2, v, 1. 



'^ Radl, E., Untersuchungen iiber den Phototropismus der Tiere, Leipsic, 1903. 

 i^Garrey, Biol. Bull., 1904-05, viii, 79. 

 ^^Lyon, Am. J. Physiol., 1905, xii, 149. 

 ^^Loeb, Arch. ges. Physiol., 1907, cxvi, 368. 



