THE PROPERTIES OF THE HEART MUSCLE 1 91 



Rhythmicity. The property of rhythmic contraction is sho\vn by the 

 action of the heart within the body; its systole is followed by its diastole in 

 regular sequence throughout the life of the individual. The force and fre- 



FIG. 168. FIG. 169. 



FIG. 168. The Heart of a Frog (Rana esculenta), from the Front. V, Ventricle; Ad, 

 right auricle; As, left auricle; B, bulbus arteriosus, dividing into right and left aortae. 

 (Ecker.) 



FIG. 169. The Heart of a Frog (Rana esculenta), from the Back. s. v. t Sinus venosus 

 opened; c. s. s., left vena cava superior; c. s. d., right vena cava superior; c. i., vena cava 

 inferior; v. p., vena pulmonales; A. d., right auricle; A. s. left auricle; A. p., opening of 

 communication between the right auricle and the sinus venosus. X 2^-3. (Ecker.) 



quency of the systole may vary from time to time as occasion requires, but 

 there is no interruption to the action of the normal heart or any interference 

 with its rhythmical contractions. Further, in an animal rapidly bled to 

 death, the heart continues to beat for a time, varying in duration with the 



FIG. 170. Automatic Contractions of Sinus Muscle from the Terrapin's Heart in 0.7 

 per cent. Sodium Chloride. Time in minutes. (New figure by L. Frazier.) 



kind of animal experimentally dealt with and depending on whether or not 

 there is entire absence of blood within the heart chambers. Furthermore, 

 if the heart of an animal be removed from the body, it still continues its 



