338 



PRACTICAL PHYSIOLOGY 



CHAPTER XXI. 

 THE HEART CONTINUED. 



The Contraction Curve and 

 Latent Time of the Stanniused 

 Heart. Expose the heart of 

 a pithed frog. Pass a ligature 

 under the two aortae, and 

 draw the ends exactly round 

 the white crescentic line which 

 marks the sino-auricular junc- 

 tion. Tie the ligature. The 

 sinus continues to beat, while 

 the auricles and ventricle 

 stand still. Pulling the heart 

 up by the ligature, cut widely 

 round the sinus and excise the 

 heart. Place it beneath the 

 heart lever (Fig. 59). 



Two needle electrodes are 

 inserted into the auricles, 

 one on either side of the 

 heart. The drum is set at a 

 moderately fast rate, and the 

 trigger key is placed in the 

 primary circuit. A short cir- 

 cuiting key is placed in the 

 secondary circuit, and the 

 coil is arranged to give a 

 break shock just perceptible 

 to the tongue. Close the 

 short circuiting key, and set 

 the drum so that the striker 

 is just beyond the trigger 

 key. Then close the latter. 

 Place the lever at a tangent 

 to the drum, and bring the 

 writing point lightly in con- 

 tact. Then open the short 

 circuiting key and start the 



