I6 4 



THE CIRCULATION OF THE BLOOD AND LYMPH 



contractions of the auricular muscle when the rhythm is slowed, 

 but not otherwise. It is probable that this is due to the beneficial 

 influence of the longer period of rest associated with the diminution 

 in frequency of the beat before the inhibitory action has had time 

 to cause depression in the amplitude (Wiggers). 



Fig. 74. Influence of the Vagus on the Contraction of the Dog's Right Auricle 

 The two upper curves are simultaneous records of the contraction of very short 

 portions of the auricle (so-called fractionate contraction) taken from two 

 regions, one near the sinus node P, and one far from it, D. The contraction 

 begins and ends slightly sooner in the proximal than in the distal region. 

 The lowest curve shows the depressant effect of the vagus excitation mani- 

 fested when an artificial rhythm (a series of electrical break shocks), which is 

 not altered by stimulation of the vagus, is substituted for the normal rhythm 

 sustained by the ' pacemaker.' 



The inhibitory fibres, then, influence the heart particularly 

 through the auricles; they are par excellence auricular nerves. On 

 the other hand, the accelerantes in all mammals which have been 

 investigated not only extend to the ventricles, but are even mainly 

 distributed to them. They are emphatically ventricular fibres, and 



