MUSCULAR TWITCHINGS 761 



of the solutions is first brought in contact with the feet 

 of the frog. If the feet are not withdrawn, the next 

 stronger solution is used, and, if no reaction occurs, the next 

 stronger. If one thus succeeds in keeping the feet of the 

 animal for one minute or more in the A1C1 3 solution, subse- 

 quent contact of the feet with common tap- water or distilled 

 water makes the animal act as if the water caused the most 

 excruciating pain. The feet are violently withdrawn, rubbed 

 against each other in a way that one notices otherwise only 

 when the feet are dipped into strong acids. If the A1C1 3 

 solution chosen is too strong, the animal will not leave his 

 feet in the solution, but will try to withdraw them. But in 

 that case its attempts at withdrawing its feet from the solu- 

 tion are never as violent as the subsequent attempts at with- 

 drawing the feet when brought into contact with common 

 water. The stronger the solution of A1C1 3 is in which the 

 feet had been kept, and the longer they had been kept in 

 the solution, the stronger their sensitiveness toward water 

 will become. 



Sodium citrate acts very similarly to aluminium chloride. 

 As the latter is slightly acid and sodium citrate slightly 

 alkaline, the possibility was suggested that the H and HO 

 ions are responsible for the hypersensitiveness. While it is 

 possible to produce occasionally a slight hypersensitiveness 

 toward common water by a pure solution of NaOH or HC1, 

 the results are very unreliable. It is practically the same if 

 one tries to use NaCl solutions to which slight and varying 

 quantities of HC1 or NaOH have been added. Better results 

 can be obtained with solutions of oxalates, sulphates, car- 

 bonates, and phosphates. The sodium salts are preferable 

 to the potassium salts, for the animal withdraws its feet 

 much more rapidly from the solution of a potassium salt 

 than from the solution of the corresponding sodium salt. 

 This makes it difficult in the case of potassium salts to 



