130 EXPERIMENTAL PHYSIOLOGY 



CHAPTEE XII 



ACTION OF DEUGS UPON THE FROG'S HEART 



ACTION OF MUSCARINE AND ATROPINE 



Experiment 1. — Take a piece of fairly wide glass tubing and draw it out 

 at one end to form a pipette with very fine orifice. Arrange the apparatus 

 for recording the heart beats by the suspension method, having dissected 

 out the vagi. Record the effect of stimulating the vagus and the sinus. Take 

 some of the muscarine solution ' in the pipette and allow it to fall, drop by 

 drop, on to the heart while it is still recording. Almost at once the beat 

 becomes slower, and gradually force and rate decrease until the heart comes 

 to complete standstill in diastole. Stimulate the heart, either mechanically 

 or electrically. It will be found to require a very strong stimulus to make 

 it respond. After allowing it to remain at rest for a short time to see that 

 there is no tendency to recovery, wash out the pipette and fill it with the 

 atropine solution.' 1 Let the solution fall onto the heart. Gradually the heart 

 begins to beat again, and shows precisely the same phenomena as after inhibi- 

 tion by vagal stimulation. If the heart had been beating weakly before the 

 application of the muscarine solution, its beats after the application of the 

 atropine often attain a much greater amplitude. 



Next place one of the vagi on the electrodes and tetanise it. No slowing 

 nor inhibition occurs. Next apply the electrodes to the crescent ; still no 

 inhibition takes place. The atropine has paralysed the inhibitory nerve 

 terminals in the heart substance. The fact that atropine abolishes the mus- 

 carine effect proves that muscarine also acts on the nerve mechanism, 

 and not directly upon the heart muscle. 



In fig. 103 a record taken during such an experiment is given. The 

 solution of muscarine was applied after the fifth beat of tracing i, and 

 very quickly a change in the rhythm of each beat was produced. The 

 auricular beat, which previously commenced during the ventricular dia- 

 stole, was delayed and became less forcible. The sinus contraction also 

 became marked on the tracing. Gradually the force of the auricular 

 beat became less and less, though for a time that of the ventricular beat 

 was maintained. Later, the ventricular contraction became less forcible 

 and slower, and finally suddenly ceased. The line seen at the end of 

 the tracing shows undulations which were due to the sinus beat. 



1 Made by adding a drop of a strong muscarine solution to some normal saline 

 solution. 



- A | per cent, solution of the sulphate in normal saline. 



