49 



B. To STUDY THE NERVE CONTROL or THE HEART. 



1. Intra-Cardiac Mechanism. Fix one pair of electrodes 

 from the commutator by means of a pin to the cork so that 

 their points touch the crescent, which may be seen as a white 

 crescentic mark between the sinus and auricles on their 

 posterior aspect (fig. 14). and, with the point of the lever 

 swung off the drum by means of the base-piece, stimulate 

 by closing the key in the primary circuit and opening that in 

 the secondary. The current must not be very strong. If no 

 change in the rate of the heart occurs, increase the strength 

 of the current till a change occurs. Don't continue to stimu- 

 late after an effect is produced. At once close the key in the 

 secondary. Now swing the lever on and take a tracing. 

 Let the drum run till the rate of the heart is restored. 



What CONCLUSION do you draw ? 



2. Extra-Cardiac Mechanism. Thrust the electrodes con- 

 nected with the other side of the commutator into the 

 medulla, fixing them on the cork of the frogboard by a pin. 

 Tilt the bridge of the commutator to send the current 

 through them, and with the lever off the drum stimulate 

 and, if necessary, increase the strength of the current till a 

 distinct effect is produced, stopping the stimulation whenever 

 this occurs. Then swing on the lever. Again stimulate and 

 take at race, allowing the drum to run till the rate of the heart 

 is restored. 



What CONCLUSION do you draw ? 



3. Effect of Drugs. Leaving the electrodes in position as 

 in 1 and 2, paint the heart with O'l per cent, solution of 

 atropine sulphate. Allow two minutes to elapse and then 

 stimulate (1st) the crescent, (2nd) the medulla. Note any 

 difference from the previous reaction. Take a tracing and 

 formulate your conclusion. 



4. What is the Influence of the Sinus ? Stannius Ex- 

 periment. By means of a needle, pass a piece of thread 

 under the two aortae, and, turning the ventricle forward, 

 tie a loose loop between the auricles and the sinus 



