118 



PRACTICAL PHYSIOLOGY 



Secondary Tetanus. If the nerve be stimulated with a rapid 

 series of induction- shocks the muscle B goes into tetanus and its 

 " currents of action " stimulate the nerve A, with the result that 

 the tetanus is also observed in the muscle A. This " secondary 

 tetanus " can be produced by rapid mechanical stimuli. 



Secondary Twitch from the Heart. If a freshly prepared and very 

 excitable nerve be laid upon the heart of a frog, 1 so that the cut end 

 of the nerve is on the base and the longitudinal surface upon the 

 apex of the ventricle, a twitch of the muscle connected with the 

 nerve is observed at each contraction of the ventricle. Each time 

 the muscle-fibres of the ventricle contract, a " current of action " is 

 produced and stimulates the nerve. 



A fine glass rod should be placed under the middle portion of the 

 length of nerve, which lies on the ventricle, so that the current may 

 not be short circuited. 



FIG. 118. Diagram of the experiment to show the stimulation of a 

 muscle by the " current of action " of another muscle. 



Stimulation of a Muscle by the " Current of Action " of another 

 Muscle. The sartorius muscle is very carefully dissected on each 

 side, and then the one muscle is placed overlapping the other ; the 

 contact of the two muscles is secured by gentle pressure with two 

 pieces of cork (Fig. 118). Stimulation of one muscle will produce a 

 contraction in both ; the " current of action " in the first stimulates 

 the second muscle. 



FIG. 119. Diagram of the experiment to show the stimulation of a 

 nerve by its own " current of injury." 



Stimulation of a Nerve by its own " Current [of Injury." Two 

 plugs of kaolin moistened with normal saline solution are placed 

 upon a piece of glass, and the tails of the plugs are made to hang 



1 For these preparations the frogs should have been kept cold for some 

 time before the experiment. 



