152 ADAPTATIONS TO ARHYTHMIC ACTIVITY 



one can say. Contractile ones look no different from non-contractile; 

 and there are such paradoxical situations as that in the frog, where the 

 common 'red' rods, with stubby myoids, have a respectable migration 

 while the scanty 'green' rods, with very long myoids like a teleost rod 

 or a dark-adapted frog cone, move but little if at all (see Fig. 23, p. 55). 



The photomechanical phenomena are interesting enough and baffling 

 enough in their normal operation, but perhaps the most remarkable thing 

 about them is that they may actually work better when out of their 

 normal environment : it has been found that the pigment and cone move- 

 ments become almost twice as extensive in a salamander eye which has 

 been transplanted, in the larva, into the location of the ear! 



As to the control of the movements, opinion is divided between two 

 schools of thought. Most of the work — not all — on fish material indi- 

 cates that efferent nerve impulses control the movements. Cutting one 

 nerve to the eye may halt the migrations, and cutting another in addition 

 may let them recommence. In one fish, the day-night rhythm of the retina 

 is known to persist even in constant darkness; nor does a frog stay dark- 

 adapted in darkness — in a few hours the cones elongate, then shorten. 



Much of the reported work on amphibian material favors the idea of 

 control by blood-borne substances. Nerve-cutting has little effect, vessel- 

 ligation a considerable one. The transplanted eyes referred to above were 

 entirely divorced from nervous control, but adequately vascularized. The 

 effects of drugs, narcotics, and anaesthetics are ambiguous. The effects 

 of temperature are especially mysterious, for both high and low tempera- 

 tures cause light-adaptation in the dark and inhibit the dark-adaptation 

 of previously light-adapted animals. Excised dark-adapted eyes will light- 

 adapt readily, but excised light-adapted eyes will dark-adapt only in the 

 surprisingly special environment of a second frog's body cavity. Emo- 

 tional states interfere with the phenomena, but these could have their 

 effects by means of either nerve-impulses or hormones. Yellow light is 

 more effective than other colors, speaking either for a reflex control from 

 the visual apparatus itself (as in the case of iris reactions in higher ani- 

 mals) , or for a direct influence of acid, formed maximally in the retina 

 under yellow light. On the other hand, old evidence — still commonly 

 cited — for reflex effects from one eye to the other and from the skin to 

 the eyes is under grave suspicion, for in the experiments the animal's 

 breathing was interfered with and fear was introduced, by the employ- 

 ment of leather hoods. Either of these factors is now known to be 

 enough to make a frog light-adapt, though he be in darkness. 



