THE CIRCULATION OF THE BLOOD 



75 



(12) If the auricles beat, divide them. If they continue to beat, 

 do they follow the same rhythm? 



(13) If the ventricle becomes quiescent, stimulate it either mechan- 

 ically or with a single induction shock. How does it respond to 

 a single stimulus? Continue to subdivide the heart until the parts 

 refuse to respond to stimuli. 



(14) Repeat the experiments and see if the results are the same 

 on subsequent trials. Note results and give your interpretation. 



C. To Make a Graphic Record of the Frog's Heart Beat. 



1. Appliances. Large frog; kymograph; heart lever. (For 

 description of heart lever see Appendix, 10.) Frog-board myo- 

 graph or similar apparatus; chronograph with a chronographic 

 system, adjusted to record seconds upon the kymograph; cover- 

 glass; normal saline; operating case. 



FIG. 43 



Frog-heart lever : t, tripod to support lever ; p, the pivot ; c, counterpoise ; h, frog's heart, 

 on which the cork point rests. 



2. Preparation. Pith a frog without destroying its spinal cord. 

 Take great care not to cut a vertebral artery during the pithing 

 operation. Should a hemorrhage occur plug the opening with 

 absorbent cotton. Hemorrhage depletes the circulatory system, and 

 the action of the heart is weakened. 



In the operation to expose the beating heart, take care not to cut 

 any large vessel, for the reason just given. Pin out the frog, ventrum 

 up, upon the frog-board myograph. Expose the heart as described 

 in the previous lesson. Open the pericardium carefully, thus expos- 

 ing the heart to direct observation. Place some resistant object 

 a cover-glass, for example under the ventricle. So adjust the 

 heart lever that the wedge-shaped cork foot of the long arm of the 

 lever will rest upon the groove which marks the line of juncture 

 between the auricle and the ventricle. (See Fig. 43.) If the weight 



