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THE CIRCULATION OP THE BLOOD 



but strengthens the ventricular beat. It is clear, however, that the 

 strengthening of the ventricular beat is merely due to the fact that the 

 cavity has become better filled with blood during diastole as a result of 

 the slowing of the auricle. These results indicate, then, that with weak 

 stimulation the vagus exerts its direct influence only on the auricle. If 



Fig. 64. Simultaneous tracings from auricle and ventricle of turtle's heart. Between the crosses 

 the vagus was stimulated, with the effect that the auricular beat diminished in force but not in 

 frequency, while the ventricular beats were practically unaffected. (From Howell's Physiology.) 



the stimulation is strong enough both auricles and ventricles cease to 

 beat altogether, and if the stimulus is maintained, the inhibition may go 

 on for a very long time (Fig. 65). 

 Usually, even though the stimulus is maintained the heart begins to 



Fig. 65. Effect of vagus stimulation on heart of turtle. Note the after effect of augmentation. 



beat again "after a time, at first only occasionally but gradually more 

 rapidly. This is known as escapement, and it indicates that the energy 

 pent up in the heart during the vagus inhibition has at last overcome 

 the inhibiting influence of the nerve, which is meanwhile becoming 

 fatigued. All of these results could be quite satisfactorily explained on 

 the assumption that the action of the vagus is confined to the sinus, 



