1016 PHYSIOLOGY 



their fibres. Under such circumstances 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 occa- 

 sionally one may observe a reversed beat, i. e. a contraction of the auricle 

 following instead of preceding each ventricular 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 tissile 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 nega- 

 tively 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 a difference in the nature of the 

 impulse as to a difference in the place of incidence of the impulse. 



Thus, if the vagus fibres which are distributed to the remains of the sinus are 

 specially active, we shall get alterations of rhythm affecting the whole heart. If those 

 which supply the A.V. bundle are excited, the most pronounced effect will be on the 

 propagation of the excitatory process from auricles to ventricles. 



Practically the same description will apply to the action of the vagus 

 on the frog's heart. Since it is easy in this animal to register the contrac- 

 tions of the empty heart, it is possible to show that the vagus has a direct 

 inhibitory action on the ventricles, diminishing the strength of its contrac- 

 tion in response to the stimuli transmitted to it from the venous end. This 

 action of the vagus on the ventricle is not however universal, and in the 

 tortoise it is impossible to show any such action. In both these animals 

 the auricles show the same effects as in the mammal, viz. an influence 

 limited to the rhythm when only the sinus is affected, or a diminution of the 

 strength of contraction when the sinus is unaffected and the chief action of 

 the vagus is on the auricular muscle. 



Ever since the discovery in 1845 by the brothers E. H. and E. F. Weber 

 of the action of the vagus on the heart, much work has been expended with 

 a view to determining the intimate nature of the inhibitory process. In 

 the former neurogenic theory it was supposed that the vagus altered the 



