64 PEACTICAL PHYSIOLOGY 



contractions. The inhibitory effect of the first ligature has been 

 attributed by some authors to excitation of the vagus nerve. 



The Heart cannot be thrown into Complete Tetanus. Set up a 

 circuit for giving single induction shocks (see Fig. 16, p. 9). Apply 



FIG. 63. Contraction of the frog's heart recorded by the suspension method. The 

 effect of tightening the first Stannius ligature at first gently and then firmly. The 

 curve should be read from right to left. The time is marked in seconds. (L.H.) 



the electrodes to the Stanniused heart and record the effect of rapidly 

 repeated excitations. The heart gives an incomplete tetanus curve. 

 Owing to the refractory period it cannot be completely tetanised. 



Pio. 64. Effect of tetanising the Stanniused heart. The curve should be read from 

 left to right. The time is marked in seconds. The third line shows the period of 

 stimulation. (L.H.) 



The Extra-systole and Compensatory Pause. Excite with a single 

 induction shock a rhythmically beating heart. The heart is recorded 

 as in Fig. 55 or 58. An extra contraction excited during the diastolic 

 period of the rhythmically beating heart is followed by a compensatory 

 pause. Note that the heart does not respond when excited during 

 systole the refractory period (Fig. 217). 



This period of inexcitability is seen in skeletal muscle (p. 42), but 

 is of much shorter duration than the refractory period of the heart. 

 The difference probably depends upon a slower metabolism in the 

 cardiac muscle. 



