1092 PHYSIOLOGY 



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 contractions 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 contraction 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 activity, perhaps by a process of ' inter- 

 ference,' of the ganglion-cells responsible for the' origination of the 

 rhythm. Many facts, however, point to the inhibitory impulses as 

 being continued to the heart-muscle itself. Thus tetanisation of any 

 portion of the frog's ventricle, especially if it be filled with blood, 

 causes an evident relaxation of the part between the electrodes. 

 Application of nicotine to the heart prevents stimulation of the trunk 

 of the vagus from having any influence on the heart, presumably from 

 paralysis of the cells of Remak's ganglion, which lie at the termina- 

 tion of the vagus fibres, or of the synapses between the vagus fibres 

 and the ganglion- cells. It is still possible to inhibit the heart by 

 direct stimulation either of the fibres leaving this ganglion in the sino- 

 auricular junction, or of the nerve-trunks which run in the inter- 

 auricular septum. We must conclude therefore that the inhibition of 

 the heart-muscle is peripheral and depends on the direct action of the 

 nerve fibres on the muscle-cells themselves. These nerve fibres are 

 paralysed by atropine, after administration of which no inhibitory 

 effects can be produced by stimulation of nerve or muscle or any 

 part of the heart. On the other hand, muscarine apparently stimu- 

 lates the inhibitory nerve-endings, and when applied to the isolated 

 auricle or ventricle causes weakening of the beat and finally com- 

 plete inhibition, an effect which can be removed by its antagonist 

 atropine. 



Two views have been held as to the essential nature of the 



