REFLEX ACTION 13 



In the simplest cases there is movement in the single part 

 affected, as when a single tentacle of a polyp is withdrawn 

 upon being stimulated. In other cases a single stimulus 

 may produce combined movements of several parts, as when 

 the various appendages of a brainless crayfish are set in mo- 

 tion when but one of them is stimulated. A stimulus on the 

 right side of a frog may not only cause coordinated action in 

 the various muscles of the hind leg, but the right fore leg 

 may be applied to the stimulated part as well. And if 

 the animal is rendered especially responsive by the injection 

 of a solution of strychnine, contraction may take place in 

 most of the muscles of the body. 



One factor which is concerned in the complexity of the 

 response is the strength of the stimulus. A light stimulus 

 applied to the tentacle of a jelly fish usually causes a contrac- 

 tion of that tentacle only, but if the stimulus is stronger, 

 the neighboring tentacles will also contract, and if still 

 stronger, movements may be set up hi the umbrella or disk 

 which cause the animal to swim away. Complex responses 

 are also produced if a stimulus affects many sense organs at 

 once. 



A further method of complication occurs in the so-called 

 chain reflexes in which one action serves as the stimulus 

 for a second, and this for a third, and so on. A frog with 

 its cerebral hemispheres removed will respond to the move- 

 ments of a fly in its vicinity by snapping at it. This com- 

 plicated reflex causes the fly to be seized by the jaws; the 

 stimuli caused by the juices of the fly acting on the taste 

 buds of the frog's mouth set up the swallowing reflex, which 

 involves complex and coordinated movements of the tongue 

 and throat. When hi the stomach the fly reflexly excites 

 this organ to activity, resulting finally in the extrusion of the 

 digested mass into the intestine, which in turn is reflexly 



