LABORATORY WORK 225 



Action of Drugs on the Heart 



Arrange a frog heart for perfusion (p. 187). Add a small amount 

 of adrenaline to the Ringer's solution used for perfusion (one part 

 in several thousands is active). The heart beat is increased in 

 rate and in strength owing to stimulation of the accelerator 

 (sympathetic) nerve-endings in it. 



Add a very small quantity of acetyl-choline to another part of 

 the Ringer's solution (one part in a million or less). The heart is 

 stopped. 



Prepare another frog for recording the effect on the heart of 

 stimulation of the vagus (p. 217). Having obtained slowing or 

 complete inhibition, drop i per cent, solution of atropine sulphate 

 in Ringer's solution over the heart. Stimulate the vagus again. 

 No effect will be seen, nor even if the junction of the sinus with 

 the auricles be stimulated. The endings of the vagus nerves are 

 paralysed. 



The Beat of the Heart 



Expose the heart of a frog as previously described. 



Note that each beat consists of a cycle, beginning in the sinus, 

 followed by the auricles, ventricle, and bulbus in turn. 



To show that the beat is initiated by the sinus, which thus 

 controls the rate, Gaskell's method of local warming may be 

 used. Take two pieces of copper wire of about y\ in. diameter. 

 Solder flexible electric light cable on to each, so that they may 

 be joined up to a storage cell, placing a key in the circuit. Wrap 

 thread around one of the wires up to half an inch of the free end, 

 and then tie the wires together. Bend each wire outwards at the 

 free end, so that there is a gap of about a quarter of an inch. Solder 

 across this gap a bit of fine German silver wire. When the circuit 

 is closed, the fine wire is heated. Take care that it does not get 

 hot enough to melt the solder. It will be better to have an adjust- 

 'able resistance in the circuit. 



Having prepared a frog with the heart joined to the lever as 

 on p. 217, fix the heating wire on a pillar of plasticine so that it is 

 near to the sinus, but not touching it. On closing the circuit, the 

 rate of the heart beats increases markedly. Now move the heating 

 wire to lie just above the apex of the ventricle ; the fine wire 

 may be bent concave to escape the suspension thread. When the 

 heating current is put on there is no change in the rate if the sinus 

 is protected. But the ventricular beats decrease in amplitude. 

 This last fact is another aspect of the temperature coefficient of 

 surface energy which we saw exemplified in the effect of heat on 

 the tonus of smooth muscle (p. 196). 



