1094 PHYSIOLOGY 



THE TONIC ACTION OF THE VAGUS 



If both vagi of a mammal be divided, the heart as a rule beats more 

 frequently, showing that under normal circumstances tonic impulses 

 are constantly descending the vagi and holding the heart's action in 

 check. The extent of the quickening which is produced by section 

 of the vagi varies in different animals and is apparently associated with 

 the conditions of life of the animal and its powers of carrying out pro- 

 longed muscular exertions. Thus in the dog or horse the pulse, 

 which is normally slow, may be doubled in frequency by section of the 

 vagi. In the rabbit, which has a frequent pulse, and is only able to 

 run for a short distance, division of both vagi causes very little altera- 

 tion in the pulse-rate. It is stated that the tonic action of the vagi is 

 much greater in the hare than in the rabbit. 



This tonic action may be increased by various conditions of the 

 blood, e.g. the presence of drugs, such as morphia. 



FIG. 450. Tracings of ventricular (upper curve) and auricular 



contractions (lower curve). 

 From xtoy the accelerator nerves stimulated. Lowest line = seconds. 



ACTION OF THE SYMPATHETIC CARDIAC NERVES 

 Stimulation of the sympathetic cardiac nerves at any part of their 

 course has an effect on the heart the exact reverse of that produced by 

 stimulating the vagi. In most cases the pulse frequency is increased 

 in consequence of the action of these nerves on that part of the heart 

 from which the rhythm starts. The frequency which is attained by 

 maximal stimulation of the accelerator nerves is independent of the 

 previous rate of the heart-beat. The increase in rate involves a shorten- 

 ing of the time of the cardiac cycle, which chiefly affects the diastolic 

 period. The size of the auricular and ventricular contractions may be 

 increased at the same time as their rate. In fact, like the vagus nerves, 

 the sympathetic fibres of the heart can influence either rhythm or 

 strength of contraction, or the conduction from auricle to ventricle, 

 according to the part of the heart-muscle which is affected. 



The augmentor effect on the strength of the ventricular beats is 

 often very marked. The sympathetic fibres are much less easily tired 

 than the vagus fibres, and have a longer latent period. Whereas the 



