Cll. XI.] 



THE RHEOSCOPIC FROG 



127 



In other fishes the electric organ is composed of modified skin glands. 

 But in each case the electric discharge is the principal phenomenon 

 that accompanies activity. 



The Rheoscopic Prog. 



The electrical changes in muscle can be detected not only by 

 the galvanometer and electrometer, but also by what is known as 

 the physiological rheoscope ; this consists of an ordinary muscle-nerve 

 preparation from a fresh and vigorous frog. The nerve is stimulated 

 by the electrical changes occurring in muscles, and the nervous 

 impulse so generated causes a contraction of the muscles of the rheo- 

 scopic preparation. The following are the principal experiments that 

 can be shown in this way : 



1. Contraction without metals. If the nerve of a nerve-muscle 

 preparation A is dropped upon another muscle B (fig. 146) or upon 



FIG. 146. Galvani's experiment without metals. 



its own muscle, it will be stimulated by the injury current of the 

 muscle on which it is dropped, and this leads to a contraction of the 

 muscle (A) which it supplies. The experiment succeeds best if the 

 nerve is dropped across a longitudinal surface and a freshly made 

 transverse section. 



2. Secondary contraction. This is caused by the current of 



FIG. 147. Secondary contraction. (After Waller.) 



action. If, while the nerre of A is resting on the muscle B (fig. 

 147), the latter is made to contract by the stimulation of its 

 nerve, the nerve of A is stimulated by the electrical variation 

 which accompanies the contraction of the muscle B, and so a con- 

 traction of muscle A is produced. This is called secondary con- 



