1 92 BIOLOGICAL LECTURES. 



which affects our eyesight. The cause which lies at the basis 

 of muscular contraction is essentially of the same nature as 

 that which produces the phosphorescence in several organisms. 



Every time our muscles are put in a state of contraction 

 there takes place a phenomenon which is analogous to the 

 emission of light. 1 If we do not see the flash of light in the 

 contraction, the explanation perhaps lies in the nature of our 

 visual organs with a comparatively limited range of visual re- 

 action. 



It is theoretically possible that all living protoplasm is phos- 

 phorescent, and that there may exist some organisms endowed 

 with different kinds of visual organs, to which organisms like 

 ourselves may appear luminous in the dark, especially when our 

 muscles are in motion. And that only when the photogenic 

 property becomes grossly exaggerated, as in comparatively few 

 representatives in each of different phyla, of living organisms, 

 does our eye recognize them as such. The true physical basis 

 of phosphorescence finds its closest analogue in the common 

 phenomena of heat-production, and is as extensive as life itself. 



1 Engelmann (Ursprung der Muskelkraft, 1893, p. ro), for example, stimulated 

 the muscle in the dark, until the state of tetanus was induced, thereby hoping to 

 see the emission of light from those heat-producing particles, which may become 

 incandescent at the height of their combustive process. His result was negative, 

 however; but, as he says elsewhere (" Die Purpurbacterien und ihre Beziehungen 

 zum Licht," Bot. Zeit., 46 Jahrg. 1888, Reprint, p. 17), if it were not for the fact 

 that such thermogenic particles are so very minute and comparatively scarce, one 

 would be able to see the scintillations emanating from the inner recesses of cell- 

 substance, on account of the enormous heat developed in the combustion of such 

 particles. 



The position assumed in the present paper is, that when contraction is accom- 

 panied by phosphorescence, which is the emission of light without any sensible 

 heat, there may be two kinds of oxidizable substances very nearly related to each 

 other genetically, one of which gives rise to heat, and the other to light. The 

 assumption that there exist particles which give rise practically to light, and to 

 light alone, is supported by the fact that such particles form the physical or ana- 

 tomical basis of phosphorescence in fireflies and glowworms. 



If the facts and inferences presented in the present paper are somewhat for- 

 eign to Engelmann's theoretical conception of the relation of protoplasmic con- 

 traction and the emission of light, they nevertheless seem to me to throw a strong 

 side-light on the correctness of the fundamental assumptions in Engelmann's 

 theory of muscular contraction. 



