PEARSE. — THE REACTIONS OF AMPHIBIANS TO LIGHT. 199 



istic reactions when it was touched gently on the foot or tail, or when 

 cotton which had been moistened in ten per cent acetic acid was placed 

 in the water near it. When it was turned on its back, the righting 

 reaction occurred, though this was accomplished with some difficulty. 

 Light, however, called forth no response, even when a condensing lens 

 was used to bring the rays to a focus on the skin. The investigations 

 of Schrader ('87) and Loeser (:05) have demonstrated the fact that the 

 mesencephalon exerts an inhibitory influence on those reflex actions 

 that take place through the spinal nerves. These observers found 

 that frogs were more responsive to external stimulation after the brain 

 had been excised so as to leave only the myelencephalon than when 

 such an operation did not include the mesencephalon. In other words, 

 the midbrain had an inhibitory action on the reflexes controlled by the 

 portions of the brain posterior to it, and when the more anterior brain 

 regions (which originate the " spontaneous " reflexes) had been re- 

 moved it rendered the frogs unusually sluggish. It is probable that 

 the mesencephalon exerts a similar influence in other amphibians, 

 and that the lack of responsiveness in Necturus was due to inhibition 

 rather than lack of ability to respond to light. The following experi- 

 ments support this view. 



The portions of the brain anterior to the metencephalon were re- 

 moved in two specimens of Cryptobranchus. Both these individuals 

 were restless and usually continued to move about slowly for some time 

 after locomotion had once been induced by any form of stimulation. 

 When either of them was kept in dim light for an hour or two, however, 

 it became quiet, and, if it was afterwards suddenly illuminated (with 

 light having an intensity of about a thousand candle-meters), there 

 was in most cases an active locomotor response and the movement 

 continued for some time, even after the light had been shut off. 



As the metencephalon is poorly developed in all amphibians, and as 

 it has been shown to exert little, if any, influence on their ability to 

 perform locomotor reactions, it is safe to conclude that the myelenceph- 

 alon and the cord are the only portions of the central nervous system 

 which are essential for the photic responses. 



III. DISCUSSION AND CONCLUSIONS. 



Photic responsiveness is a quality which is probably present in all 

 amphibians, for the sixteen species which have been found to give re- 

 actions to light include representatives of most of the families of the 

 class. Light has an orienting influence on all the species which have 

 been studied ; the Caudata are mostly negative in their phototropism, 



