292 bulletin: museum of comparative zoology. 



6. The same sort of reactions were obtained when only the eyes 

 were exposed, as when the whole body was exposed, but there was a 

 considerable decrease in the sensitiveness of the skin to differences in 

 wave-lengths, compared with that of the eyes, or of the eyes and the 

 skin. 



IV. Discussion. 



Sensitiveness to differences in wave-lengths is evidently a quality 

 residing, not only in the eyes of the toad, but in the skin as well. It 

 has been shown that the reactions when the skin alone was exposed 

 to the lights were essentially the same as those when the eyes, or when 

 the eyes and the skin, were exposed. It is necessary, therefore, to 

 consider somewhat the nature of the photoreceptors in the eyes and 

 in the skin. In the eye there is the retina, with its rod- and cone- 

 cells, i. e., nerve-terminations differentiated for the reception of ether 

 waves, which set up chemical changes in the rods and cones, and thus 

 give rise to nerve impulses that are transmitted to the brain, and there 

 perceived as light. In the skin are found the terminations of the spinal 

 nerves. We are able to form some idea of how different lights appear 

 to the toad when received through the eyes, but when we attempt 

 to consider how they would appear when they are received through 

 the skin alone, the problem becomes much more difficult. It has been 

 found that in some amphibians there is a connection between the nerve 

 terminations of the eyes and those of the skin, as is to be inferred from 

 results when the skin is illuminated. Engelmann ('85) foimd that 

 changes took place in the retinae of frogs when only the skin was 

 exposed to light; though Pick ('90) obtained results which led him to 

 conclude that interference with the normal respiration was the cause 

 of these changes. Koranyi ('92) noted that illumination of the skin 

 of the frog caused microscopic changes in the retina similar to those 

 produced by the illumination of the eye itself. 



It is generally believed by modern physiologists that the perception 

 of color is a function of the cones alone, and that the rods are sensitive 

 only to light and darkness ; and that, by virtue of their power of adap- 

 tation in the dark, through the regeneration of visual purple, they 

 form the special apparatus for vision in dim light. The generally 

 accepted theories of color-vision all presuppose the existence of photo- 

 chemical substances in the eye, which, when acted upon by the different 

 wave-lengths of the visible spectrum, undergo different chemical 

 changes, which give rise to different nerve-impulses that are trans- 



