106 



VayUf 

 and.V.B 



Fig. 26 

 Fig. 26. — Cardiac Nerves of the Frog. (Foster). 



the trunk of the vagus, stimulate it with an interrupted cur- 

 rent, and observe that the vphole heart is arrested in diastole. 



If the stimulation is kept up the heart will finally " escape " 

 from the influence of the stimulus and will recommence beat- 

 ing-. Note if the auricles appear to be more inhibited than 

 the ventricle. 



There is often a difference in effect between the two vagi. 

 Sometimes one vagus is found not to possess any inhibitory 

 fibers, in which case the opposite vagus is usually found 

 especially active. It is generally found that the effect is not 

 identical on the two sides, one usually being more powerful 

 than the other. 



The arrest, or period of inhibition, is manifest in the curve 

 by the lever recording merely a straight line. If the laryn- 

 geal muscles contract, and thereby affect the position of the 

 heart, divide the laryngeal branches of the vagus. 



There is an appreciable time or latent period, before the 

 heart reacts to the stimulus and likewise when the stimulus 

 is removed the heart does not at once regain its normal move- 

 ment. .Note that when the heart begins to beat again the 

 beats are at first small and gradually rise to normal. In some 

 instances, however, they are more vigorous and quicker. Cut 

 both vagus nerves and compare the tracing with those just 

 obtained. 



