ADVANCED EXPERIMENTAL PHYSIOLOGY 



331 



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

 excitable nerve be laid upon the heart of a frog, 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 pro- 

 duced 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. 208. 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. 208). Stimulation of one muscle will produce 

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

 the second muscle. 



FIG. 209. Diagram of the experiment to show the stimulation of a nerve by its own 

 ' ' current of in j ury. " 



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 over 

 the edge (Fig. 209). The sciatic nerve of a pithed frog is carefully 

 dissected down to the knee, the thigh is cut across, but the leg and. 

 foot are left intact. The nerve is so placed that its cut surface is 

 upon one plug and its longitudinal surface upon x the other plug. A 

 watch-glass filled with strong saline solution, which is a good con- 

 ductor of electricity, is suddenly brought in contact with the ends of 



