CHAPTER XVI 

 THE PHYSIOLOGY OF THE HEART 



The Heart as a Muscle. The properties of the cardiac muscle 

 may be studied either on the beating or on the still heart. The excised 

 heart continues to beat for some time outside the body, and has the 

 power of rhythmic automaticity. The still heart is obtained in the 

 mammal when the heart is cut out from the body and kept in physio- 

 logical saline until it ceases to beat. 



The contraction of the heart is usually recorded by means of. a 

 lever which Avrites on a smoked surface (Fig. 34). By this means a 



FIG. 34. LEVER FOR RECORDING THE FROG'S HEART. (Pembrey and Phillips.) 



record such as Fig. 35 is obtained. If an arrangement of two levers 

 be used, the contractions of the auricle and ventricle can be separately 

 recorded (Fig. 36) if the heart be clamped at the auriculo-ventricular 

 groove. 



The excised heart of the frog will beat for days in a moist chamber. 

 The auricles and ventricles stop beating when a ligature is tied around 

 the sinu-auricular junction (the first Stannius' ligature) (Fig. 35). Such a 

 stilled frog-heart responds to a single stimulus mechanical, thermal, 

 chemical, or electrical by a single beat. The electrical stimulus is 

 generally chosen, but it is important to note that heart -muscle responds 



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