PEARSE. — THE REACTIONS OF AMPHIBIANS TO LIGHT. 197 



results, it was determined to make cuts in various regions of the cord 

 in different animals and determine whether the individuals thus treated 

 would show differences in their behavior. The eyes were accordingly 

 removed from four specimens of Necturus, and the cord was cut behind 

 the fourth, ninth, eleventh, and twentieth vertebras in the respective 

 individuals. All these animals gave marked reactions to light when 

 the illumination was anterior to the cut in the cord, but no responses 

 were obtained from the region posterior to this cut, even when a strong 

 beam of light was focussed on the skin. The regions posterior to the 

 cut were, however, influenced by certain forms of stimulation, and re- 

 sponded by making withdrawing movements when they were stroked 

 with a brush, or when cotton saturated with ten per cent acetic acid 

 was placed in the water near them. All the individuals seemed to stand 

 the operation well ; the gill movements continued in a normal manner, 

 and walking was carried on by the front legs, while the posterior part 

 of the body dragged behind. All these animals lived more than five 

 days, and one of them (with its cord cut behind the eleventh vertebra) 

 lived thirty-six. This particular individual was extremely active, and 

 when the front part of the body was in motion the hind legs also made 

 walking movements, though they had a slower rate than that of the 

 front legs. Furthermore, by gently pinching the tail the hind legs 

 could be induced to walk when the front legs were quiet. In swim- 

 ming, however, the trunk muscles of the whole body moved together. 

 Loeb (:03) noted similar correlated swimming movements in Ambly- 

 stoma larvae after the cord had been transected. Notwithstanding 

 such correlated movements, it may be said of the four specimens of 

 Necturus that the parts of the body in front of and behind the cut 

 in the cord carried on reactions more or less independently, and that 

 the regions anterior to this cut responded to a greater range of 

 stimuli. 



As none of the spinal amphibians tested showed sensitiveness to 

 light, even when reactions were easily induced by other forms of 

 stimulation, it seems reasonable to conclude that their lack of sensi- 

 tiveness to photic stimulation was not due to the absence of receptive 

 or motor power, but to the fact that the ultimate control (centres or 

 essential portions of reflex arcs) of these reactions lies in the brain and 

 therefore anterior to the spinal cord. 



In order to discover what parts of the brain were essential for the 

 photic responses, experiments were carried out in which certain regions 

 were excised and observations made of the deficiency phenomena thus 

 brought about. The method followed was to excise all parts of the 

 brain anterior to a certain region, and to carry the regions excised 



