68 PRACTICAL PHYSIOLOGY 



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. 

 When the heart [again beats normally swing on the lever and 

 start the drum. Again stimulate and take a trace, marking the 

 moment of stimulation and of discontinuing it, and allowing the 

 drum to run till the rate of the heart is restored. Formulate your 

 conclusions. 



3. Effect of Drugs. 



Leaving the electrodes in position as in 1 and 2, paint the heart 

 with 0-1 per cent, solution of atropine sulphate. Allow two minutes 

 to elapse and then stimulate (1st) the crescent, (2nd) the medulla. 



FIG. 71. Excitation of vago -sympathetic. 



Note the after effect a staircase augmentation of the heart-beat. The stars indicate the beginning 

 and end of stimulation. The downstroke represents contraction. The time is marked 

 in seconds. (L.H.) 



Note any difference from the previous reaction. Take a tracing 

 and formulate your conclusion. Run off a time trace in seconds and 

 fix the tracings. The experiment may be tried using nicotine 1 

 in 1000 saline solution. 



Is the Heart's Action Automatic ? Excise the heart with the sinus 

 attached, and place it in a watch glass and study its movements, 

 counting the number of beats per minute. 



Influence of Temperature. Now place the watch glass upon ice 

 and observe the effect. When a marked change in the rate has 

 taken place and been recorded, remove the watch glass from the 

 ice and place it upon the palm of the hand and record any change 

 in the rate. 



The influence of temperature may be investigated before trying 

 the effects of drugs by pouring over the heart normal saline solution 



