226 THE CIRCULATION OF THE BLOOD 



is continuously stimulated, the heart is likely to remain inhibited for a 

 long time, whereas in the cat the inhibition is very quickly broken into 

 by escapement. If the tracing is taken directly from the heart, it will 

 frequently be observed in the dog that, when the escapement occurs dur- 

 ing vagus stimulation it is only the ventricle that is beating, the auricles 

 still remaining inhibited. 



If the stimulation of the vagus is discontinued after some time in an 

 animal whose blood pressure is being recorded, the pressure will not 

 only quickly recover, but will usually overshoot the normal level, mainly 

 because of the asphyxia which has been produced during the period of 

 inhibition. The asphyxia raises the hydrogen-ion concentration of the 

 blood and this stimulates both the vasoconstrictor center and the heart 

 action (page 168). The increased heart action is, also partly owing to the 

 fact that during vagus inhibition the beating pOAver of the heart becomes 

 improved (page 230). 



As an outcome of recent work, 14 it has been show 

 nerve acts mainly on the sinoauricular node, and the left vagus on the 

 auriculoventricular bundle. This is in agreement with the observations 

 described above on the cold-blooded heart (page 222). Stimulation of the 

 right vagus always causes slowing and weakening of both the auricular 

 and the ventricular beats, but stimulation of the left vagus is sometimes 

 observed to have but little influence on the auricular beat, although it 

 may produce a condition of partial heart-block; or, if a clamp is ap- 

 plied to the auriculoventricular bundle so as to produce a partial heart- 

 block, then during stimulation of the left vagus, the block may become 

 complete. There is, however, a considerable overlapping of these in- 

 fluences, at least in the case of the left vagus, for this nerve also acts 

 considerably on the ventricle, influencing perhaps not so much the rate 

 as the force of the contraction. It has been found experimentally that, 

 in order to demonstrate the specific action of the left vagus on the bun- 

 dle, it is most suitable to study the relationship between auricular and 

 ventricular beats when the auricle is beating rapidly as during the 

 application of artificial (electrical) stimuli to it. Ordinarily the con- 

 traction produced by each stimulus passes into the ventricle, but during 

 stimulation of the left vagus it is found that every contraction does not 

 pass. These experiments raise the question as to what the influence of 

 either nerve may be in blocking impulses from the auricles to the ven- 

 tricles when auricular fibrillation is present. It might be expected that 

 the left vagus would prove more effectual in this regard, but actually it 

 has been found that both vagi have the same effect. 



Tonic Vagus Action. Impulses are constantly passing along the vagi 

 to the heart. On account of this so-called tonic action, the heart rate 



