LESSON XII. GRAPHIC RECORD OF BEAT OF 

 FROG'S HEART. ACTION OF POISONS. 



1. Graphic record of cardiac inhibition. Dissect 

 out the vagus as in Less. XI. Stimulate it; if the stimu- 

 lation does not stop the heart, the nerve has probably 

 been injured and the vagus of the opposite side should 

 be prepared. 



Another way of dissecting the vagus is as follows. 



Hold the head of a pithed frog between finger and thumb. Cut 

 away the skin on the back of the head. Pass a scalpel carefully 

 along the posterior edge of the skull, so as to cut through the muscles 

 attached to it. The vagus nerve, the jugular vein, and the petro- 

 hyoid muscle will come into view, and on a little further clearing 

 of the muscles, the glossopharyngeal nerve also. Pass a threaded 

 needle under the vagus; put the frog on its side, tie the nerve and 

 cut it close to the skull. Now expose the heart. 



A convenient lever to use for the experiment is shown 

 in Fig. 21, but the straw S should be somewhat longer 

 than in the figure. Arrange the lever as in Pig. 21 (2), 

 and put it at such a height that by raising or lowering 

 the drum a tracing can be taken at any level of the 

 blackened paper without shifting the lever. Put a piece 

 of plasticine on the short arm A so as nearly to counter- 

 poise the long arm, and adjust the lever so that the 

 writing point presses lightly only on the paper. 



Pass a silk thread through the ring of a small wire 

 clamp (Fig. 23 Gl.) and tie in the thread a loop 2 to 3 

 inches long. Fix the points of the clamp in the extreme 

 tip of the ventricle, in doing this the ventricle must not 



