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A TEXTBOOK OF PHYSIOLOGY 



with a quick stimulation of the ventricles, these may beat first, fol- 

 lowed by the auricles. The rate of conduction of the excitatory 

 \wave is said to be in favour of muscular conduction. 



It is certain that the excitatory wave arises in the region of the 



FIG. 49. SHOWING EFFECT OF EXCISION OF SINU-AURICULAR NODE AND PART OF 

 THE SUPERIOR VENA CAVA AND RIGHT AURICLE AT A IN RABBIT'S HEART. No 

 STOPPAGE OF HEART. (M. F.) 



great veins, and under normal conditions passes through the auricles 

 and thence to the ventricles. With the discovery of the sinu-auricular 

 node at the junction of the superior vena cava with the auricle there 

 was a tendency to regard this nodal tissue as the automatic tissue of 

 the heart. Experimental evidence has shown that the nodal tissue 



FIG. 50. SHOWING THE EFFECT (B) OF LIGATURE OF THE MUSCULAR CONNECTION 

 BETWEEN AURICLES AND VENTRICLES IN THE HEART OF THE CHICKEN. HEART- 

 BLOCK is INDUCED. (M. F.) 



A Normal rhythm before ligature. Time in seconds. 



possesses a high degree of automaticity. The string galvanometer shows 

 that it is in the sinu-auricular node that the normal excitatory wave 

 of the heart arises (Fig. 46). Here alone can the normal rhythm of 

 the mammalian heart be modified e.g., by cold, which lessens the 

 frequency of the heart (Fig. 47), or by mechanical or electrical 

 stimulation. 



But clamping or excision of the sinu-auricular node does not stop 



