CIRCULATION OF THE BLOOD. 



chlorid solution, without previous immersion in the sodium- chlorid 

 solution. Is the muscle stimulated to pulsate ? Does calcium act 

 as a stimulus to ventricular contraction ? 



3. Combined Action of Sodium and Calcium. To a 0.7- 

 per-cent solution of sodium chlorid in a beaker add one-tenth of 

 the volume of calcium chlorid. Immerse a fresh muscle prepara- 

 tion in this solution. How does the length of time during which 

 contractions are maintained compare with that in which sodium 

 chlorid alone was used? What is the character of the individual 

 contractions ? 



4. Potassium. Immerse a strip of ventricular muscle in a 

 o.Q-per-cent potassium-chlorid solution which is nearly isotonic 

 with 0.7 per cent NaCl. Are rhythmical contractions brought 

 about ? 



5. Combined Action of Sodium, Calcium, and Potassium. 

 Immerse a strip of ventricular muscle in a solution of sodium 

 chlorid 0.7 per cent, calcium chlorid 0.025 per cent, and po- 

 tassium chlorid 0.025 per cent (slightly modified Ringer's so- 

 lution). 



Record the contractions upon a very slowly moving drum. How 

 long will rhythmical contractions continue? How do they com- 

 pare with those obtained from the muscle treated with 0.7 per 

 cent NaCl alone, and with those from the muscle immersed in a 

 combination of Na and K ? 



XI. STANNIUS' EXPERIMENT. 



Pith a frog. Carefully expose the heart. Tie the fraenum, the 

 partition of pericardium attached to the dorsal aspect of the ven- 

 tricle, and use the ligature as a guide. Pass a thread around the 

 junction of the sinus with the auricles and tie snugly (see i, Fig. 30). 



The sinus continues to beat. The auricles and ventricle stop 

 beating. Now tie a second ligature around the heart at the auric- 

 ulo-ventricular groove. The ventricle again begins to beat. The 

 auricles will probably remain quiescent. 



Tie a third ligature about the middle of the auricles, at line 2, as 



[95] 



