CIRCULATION OF THE BLOOD. 



containing both accelerator and inhibitor fibres, and the upper one 

 the laryngeal branch of the vagus. 



Trace the cardiac branch of the vagus trunk to its distribution 

 in the heart. This trunk as thus exposed in the thorax is really a 

 combined nerve, being formed by the junction of the vagus proper 

 and the cardiac branches of the sympathetic. 



VI. EFFECT OF STIMULATION OF THE VAGO-SYMPATHETIC 

 TRUNK UPON THE HEART-BEAT. 



Pith a frog. Carefully expose the heart and isolate the vago- 

 sympathetic trunk. Place this upon fine platinum electrodes, 

 avoiding, so far as possible, contact with other nerves or the sur- 

 rounding tissues. 



Connect the ventricle, by the suspension method, with a light 

 lever made to write upon a medium fast drum. 



(a) Set up the inductorium for weak tetanizing induction 

 shocks and connect the secondary with the vagus electrodes. Ar- 

 range a time-marker writing quarter-seconds to trace beneath the 

 heart-beat record. 



While the ventricular tracing is being taken, stimulate the vagus 

 with weak induction shocks for ten seconds. Is there any change 

 in the rate or strength of the heart-beat ? If no appreciable effect is 

 produced, increase the strength of the current slightly, noting the 

 distance of the secondary from the primary, and repeat the stimu- 

 lation of the nerve. 



(b) Increase the strength of the stimulus, noting the effect on 

 the heart-beat with each increase, until strong stimulation is em- 

 ployed. 



What is the effect of weak stimulation of the nerve ? Of stronger 

 stimulation ? Of the strongest stimulation which you applied ? 



What is the after-effect upon the heart-beat, following the cessa- 

 tion of the stimulation ? 



(c) Apply a strong stimulus to the nerve and continue this for 

 a minute. Does inhibition continue during the entire time of the 

 application of the stimulus ? 



