402 



GENERAL PHYSIOLOGY 



ally (Fig. 187), it can easily be demonstrated that this jumping 

 movement is not a thermal effect of light, but is caused chiefly by 

 the blue and violet rays, and, therefore, the rays that are least 

 effective thermally. The same effect can be produced by heat-rays, 

 but ordinary daylight is not sufficient for this ; it requires sunlight 

 of considerable intensity, such as can be obtained by the concentra- 

 tion of direct sunlight by means of a concave mirror. 



As regards the motion of cross-striated muscles, no instance is 

 thus far known in which light has exerted an influence upon it with 

 certainty. Nevertheless, some time ago Steinach ('92) showed 

 that certain smooth muscle-fibres can be made to contract by light 

 stimuli. In fishes and Amphibia the sphincter iridis, a muscle 

 which in contraction narrows the pupil of the eye, is, as Steinach 

 found, composed of smooth muscle-fibres which contain a brown 

 pigment. These fibres are stimulated by light directly, without the 



FIG. 187. Spectra of various media ; 1, of a red glass ; 2, of a cobalt glass ; 3, of a green glass ; 

 4, of a solution of potassium bichromate ; 5, of an ammoniacal solution of a cupric salt. 



mediation of the central nervous system ; this is proved by the 

 tact that even the excised muscle can by illumination be made to 

 contract. 



Just as in many cases contraction-movements are caused by light, 

 the peculiar motion of the Diatomece can be influenced in a certain 

 sense by the same stimulus. As Engelmann ('82) has found, this 

 ceases when the organisms are put into a dark chamber and 

 oxygen is excluded. But it immediately begins again, when light 

 is allowed to act upon them. This phenomenon, as Engelmann 

 showed, is due to the fact that, with the exclusion of oxygen, the 

 oxygen necessary to the motion of the Diatomece is soon consumed. 

 If the latter be put into darkness, their movements immediately 

 cease ; if they be brought into the light, they split up carbonic 

 acid into carbon and oxygen, by means of their yellow colouring- 

 matter, which is allied to chlorophyll, and in this manner themselves 

 produce the oxygen that is necessary to their movements. 



