STIMULATION OF THE CRESCENT 125 



but with that of the mixed vagus and sympathetic. To determine 

 which of these results are due to vagal fibres only, our only procedure 

 can be to stimulate the vagus inside the skull before it has been joined 

 by the sympathetic fibres. We may state as the general result of 

 impulses passing down the vagus fibres — 



(i.) That they cause slowing or inhibition of the beat, depending 

 upon the strength of stimulus employed. 



(ii.) That they tend to weaken the force of the beat when slowing 

 occurs. 



(iii.) That as an after-effect there is for a time an acceleration 

 accompanied by an augmentation of the force. 



There is often to be observed a difference in effect between the two 

 vagi. Sometimes one vagus is found not to possess any inhibitory 

 fibres, in which case the opposite vagus is found especially active. It 

 is usually found that the effect is not identical on the two sides, one 

 usually being more powerful than the other. 



Experiment 2. — Contrast the effect of stimulation of the crescent with 

 stimulation of the vagus. Prepare the apparatus as for the preceding 

 experiment. Dissect out both vagi, passing fine threads round them, so that 

 they may be readily picked up when required. Having attached the heart 

 to the lever place one vagus upon the electrodes, and after recording a short 

 piece of normal tracing send in a tetanising current into the vagus and record 

 its effect. Next repeat on the vagus of the opposite side. Finally apply the 

 electrodes, so that they touch the crescentic junction between sinus and 

 auricles, and record the result of stimulation in this position 



Fig. 101 gives the result of such an experiment. The first tracing 

 is interesting because no inhibition resulted on stimulating the right 

 vagus. The only alteration seen is a slight change in the sequence of 

 the beat, the ventricular contraction commencing a little earlier in 

 the auricular relaxation. Stimulation with stronger currents also 

 had no inhibitory effect. Stimulation of the left vagus produced a 

 typical inhibition (n, fig. 101). Finally in in is seen the effect of 

 stimulation of the crescent. In this latter there are but slight dif- 

 ferences from a typical vagus effect. Complete inhibition follows after 

 a short latency, and on cessation of stimulation there is after a short 

 latency a return of the beat, which shows the staircase effect, though 

 not so clearly as in n. After the stimulation the auricular con- 

 traction tends to commence earlier in the ventricular relaxation, a 

 change gradually occurring, until finally it is seen to begin at about 

 the middle of the relaxation. 



Experiment 3. — Examine the action of the sympathetic upon the heart. 

 Arrange the apparatus as in the previous experiment. Dissect out the sym- 

 pathetic on one side, placing a ligature around it, as described on p. 121. 



