EEFLEX ACTION 215 



flank ; the leg of the same side is rapidly drawn up, as if to push away 

 the object stimulating the skin. These are all instances of unilateral 

 reflex movements, and may be extended in many directions. 



(b) Pinch the skin over the anterior surface of the pubes or round 

 the anus ; both legs are now drawn up to rub the spot stimulated. 



(c) Pinch one of the toes gently. With a mild stimulus there 

 may only be a slight flexion at ankle and knee. Increase the strength 

 of the pinch, and the movements of the leg become more marked. If 

 the strength be still further increased, movements of the opposite leg 

 will also be produced. 



2. Electrical. 



(a) Single induction shocks.— Apply a pair of wire electrodes from 

 the secondary coil to the skin of the leg. Stimulate with single induced 

 shocks, gradually increasing the strength of the shock. No reflexes 

 are produced, though a twitch produced by the direct excitation of 

 the muscles may be produced if the stimulus be sufficiently strong. 



(b) Bepeated induced shocks. — At first employ very weak stimuli. 

 A reflex is quite readily obtained. This forms an instance of summa- 

 tion of effect. A single stimulus produces no result ; but if repeated, 

 even though weak, the effects are gradually summed up until they are 

 able to produce a series of reflex impulses. 



3. Chemical. 



(a) Take some 0-2 per cent, sulphuric acid in a small beaker, 

 and with the frog suspended move one leg to one side with a 

 loop of thread or a glass rod, and then immerse the other foot 

 in the dilute acid. In a short time the leg is withdrawn from the 

 acid, but will again relax, dip into the acid and be withdrawn for a 

 second time. Remove the acid and wash the skin thoroughly with 

 water to remove all traces of acid. Allow the frog to rest for a few 

 minutes. 



In all cases where a chemical irritant has been employed it is of 

 the greatest importance that this resting period should be sufficiently 

 long — five to ten minutes — before a fresh excitation is attempted, other- 

 wise the results obtained are not characteristic. 



(b) Repeat the experiment, using 10 per cent, acetic acid instead 

 of the sulphuric. 



4. Thermal. 



Touch one of the toes with a heated wire ; the foot is withdrawn. 



Having shown by these experiments that a reflex act is produced 

 in response to mechanical, electrical, chemical, or thermal stimuli, 

 proceed next to study the characters of the reflex. 



