70 PKACTICAL PHYSIOLOGY 



CHAPTER XVI. 

 THE SINO-AURICULAR JUNCTION. THE ACTION OF DRUGS. 



Inhibition Produced by Excitation of the Sino- Auricular Junction. 

 The heart is recorded by the suspension method. Observe the white 

 tendinous line which marks the sino-auricular junction. It is curved with 

 its convexity upwards. This is known as the crescent. Pin the cork of the 

 fine wire electrodes to the frog-plate so that the ends of the electrodes touch 

 the crescent. The ends must not be more than 2 mm. apart. Start the 

 drum (slow rate), record half-a-dozen beats, and then tetanise the 

 crescent. The heart, owing to direct excitation of the muscle, at first 



FIG. 71. Contraction of the frog's heart. Excitation of the sino-auricular junction. 

 Arrest of auricles and increased rate of ventricle (incomplete tetanus). A pause 

 followed the cessation of the excitation. The curve should be read from right to left. 

 The stars indicate the beginning and end of stimulation. (L.H.) 



beats faster, and then is arrested in diastole. Sometimes the arrest does 

 not take place till the excitation ceases. The heart soon escapes from 

 arrest. The arrest is due to excitation of the intra-cardiac branches of 

 the vagus. Mechanical stimulation of the ventricle during the arrest 

 will cause a reversal of the natural sequence. The sinus continues to 

 beat during the period of arrest. The excitatory wave is blocked 

 in the auricular muscle. 



Action of Muscarine and Atropine. Dissect out the vago-sympathetic 

 nerve and record the effect of excitation of (1) the vago-sympathetic, 



