The "Fly-catching Reflex" in the Frog 113 



The frog poisoned by yohimbine snaps and generally extends his tongue 

 if his hand or nose be touched ; sometimes if a bright object be brought 

 near his eyes. If the hand of a normal frog be touched the arm is drawn 

 away ; if the nose be touched the head is depressed and the eyes closed ; 

 while if a bright object be approached close to its eyes, the animal merely 

 moves away or closes its eyes if it react at all. But Schrader (1) has 

 shown that the snapping for food is a reflex from slight stimulation, and 

 that a frog deprived of its cerebrum will catch flies under suitable circum- 

 stances, and given a long enough time for recovery from operation. He 

 has also shown that if the brain is destroyed down to the fore part of the 

 medulla oblongata there is developed a somewhat difterent snap reflex. In 

 this case the frog snaps if its nose or hand be touched lightly, the head 

 being directed as far as possible towards the place of stimulation. 



The remarkable resemblance between these reflexes and those occurring 

 after injection of yohimbine led me to conclude that yohimbine, by a para- 

 Ij^sing action on the upper part of the central nervous system, imitates the 

 operative lesions and results of such lesions as Schrader describes. 



The evidence which I have cited above shows that yohimbine does 

 paralyse the functions of certain parts of the supraspinal portion of the 

 central nervous system. Though the appearance of this fly-catching reflex 

 has not to my knowledge been described hitherto as the result of the action 

 of a toxic agent on the frog, still, as a consequence of such action, symptoms 

 simulating the effects of operative lesions are well known to occur. 



However, in the case of poisoning of an intact animal, when touching 

 the muzzle or hand or bringing a bright object near the eyes evokes a fly- 

 catching or snapping reaction, there is a possibility that this reaction is 

 a voluntary one, abnormal it is true, but rendered elicitable, for example, 

 by a depression of some normal inhibitory influence of the cortex cerebri. 

 In order to decide this point I destroyed the cerebra of two frogs and 

 administered yohimbine to one of them immediately after operation. The 

 snap reflex occurred in a few hours in the poisoned frog, and in it alone. 

 Since it occurs when voluntary impulses are cut off", it is a true reflex. 



There is another possible explanation of the production of this reflex 

 bj^ yohimbine, still in keeping with the known method of action of drugs. 

 There would seem to be in this condition a hypersensitiveness of the centre 

 involved, or, what may be the same thing, a condition in which varied and 

 divergent afferent stimuli find a path of least resistance in reflexion 

 through the centre for the apprehension of food. The action of yohimbine 

 may therefore consist in " facilitation of the discharge of force already 

 latently present, and the rendering of the liberating forces more ettective 

 tending to thwart inhibition " (2), and this action may be exerted especially 

 on this centre in the bulb. Certain facts go to show that yohimbine may 

 so act on the nervous system. The fact that small doses of j'ohimbine, by 

 an action on the respiratory centre, induce an increase in the rate and 

 amplitude of the respirations (3), may be explained by the same kind of 



