THE NERVOUS REGULATION OF THE HEART 1091 



of the vagus on the auricles and ventricles will consist in an alteration 

 of rhythm. They may cease to beat altogether or they may give beats of 

 normal strength but at a slower rhythm than before. Often indeed 

 under these conditions the beats of the ventricles may be increased 

 in size, since the strength and extent of their contractions are deter- 

 mined, not by the strength of the stimulus arriving from the auricles, 

 but by the tension to which their fibres are exposed, and this will 

 increase with any lengthening of the diastolic period, and consequent 

 increased diastolic filling of the ventricle. 



If the vagus acts on the auricles without affecting the sinus part 

 of the auricles (sino-auricular node), the rhythm will be unaltered, 

 but the response of the auricles to the impulses received by them will 

 be diminished, and the amplitude of the excursions of the lever attached 

 to them will therefore be considerably reduced. Indeed the auricular 

 contractions may be reduced to such an extent that they cause no 

 movement of the lever. It is only by observing their surface that one 

 may perceive a slight contraction of their fibres. Under such circum- 

 stances the rhythm of the ventricles will be unchanged. 



Generally the vagus absolutely stops the action of all parts of the 

 auricles ; in such cases the ventricles also cease beating. Very often 

 after a short pause the ventricles commence to beat at a slow rhythm, 

 and it is then seen that they are contracting independently of the auricles 

 and sinus. That the ventricle is really inaugurating the beat is shown 

 by the fact that occasionally one may observe a reversed beat, i.e. a 

 contraction of the auricle following instead of preceding each ventri- 

 cular contraction. Whether the vagus has a direct action on the 

 mammalian ventricle is still doubtful ; its effect is at any rate very 

 slight as compared with that on the venous end of the heart. The fact 

 that stimulation of the vagus causes as a rule temporary cessation of 

 the ventricular beat, while functional separation of the ventricles from 

 the auricles causes no such temporary stoppage, would seem to indicate 

 that this nerve has a direct, though slight, action on the ventricles. 



Finally the vagus may affect the tissue which conducts the excitatory 

 process from one cavity to another. Under vagus stimulation the 

 auricles may beat at a greater rhythm than the ventricles, a block 

 having been produced in the tissue passing from auricles to ventricles, 

 viz. the auriculo-ventricular bundle. 



Engelmann has described these effects of vagus excitation as negatively 

 chronotropic (diminution of rhythm), negatively inotropic (diminished strength of 

 contraction), and negatively dromotropic (diminished conductivity), and has 

 distinguished a fourth action, viz. one on the irritability of the muscle to direct 

 stimuli, which he calls negatively bathmotropic. He ascribes these four actions 

 to four different sets of nerve fibres, but it is evident that they are due not 

 so much to the difference in the nature of the impulse as to a difference in the 

 place of incidence of the impulse. 



