THE CIRCULATION OF THE BLOOD 539 



contractions of the strip on a very slowly moving 

 drum. 



Sodium. Immerse the strip of ventricular 

 muscle in a beaker containing 0.7 per cent solu- 

 tion of sodium chloride. 



After a latent period, which may be protracted, 

 but usually is brief, a series of rhythmic con- 

 tractions will be observed. The contractions 

 soon reach a maximum and then gradually die 

 away. Sodium, although an important stimulus 

 to contraction, cannot maintain the ventricle in 

 continued activity. 



The tonus of the heart muscle is diminished 

 by sodium chloride. 



Calcium. Surround a strip of contracting 

 ventricular muscle with a solution of calcium 

 chloride isotonic with 0.7 per cent sodium chlo- 

 ride solution (approximately 1.0 per cent). 



Contractions will cease. Calcium added to 

 solutions of sodium chloride, however, will 

 lengthen the period during which the heart 

 muscle contracts and will increase the strength 

 of the individual contractions. Strong solutions 

 of calcium chloride greatly increase the tonus. 



Potassium. Surround a non-beating strip of 

 ventricular muscle with a solution of potassium 

 chloride isotonic with 0.7 per cent sodium 

 chloride solution (approximately 0.9 per cent). 



