THE CIRCULATION OF THE BLOOD. 



289 



auricles and ventricles with a clamp. By carefully regulating the 

 pressure it is possible to so block the wave that three or four 

 auricular contractions may occur before a single ventricular 

 contraction (Fig. 126). 



3. Rhythmicity. The beat of the heart is a uniform movement, 

 occurring at regular intervals. Each phase of each beat occupies a 

 regular measure of time. The beat is therefore rhythmic in char- 

 acter. The heart-muscle as a whole varies in rhythmic power in 

 its different parts. It is best developed in the frog and tortoise, 

 in the sinus venosus, less so in the auricles, least in the ventricles. 

 This may be shown by division of the tissue between sinus and 

 auricles in situ. At once the auriculo- ventricular portion ceases 

 to beat, while the sinus continues contracting as usual. In a 

 short time the auricles and ventricles begin to beat, though less 

 rapidly than formerly. Separation of the auricle from the ven- 

 tricle is again followed by rest. In due time the auricle begins 



Aur. 



Vent 



FIG. 126. RECORD or THE AURICULAR AND VENTRICULAR CONTRACTIONS BEFORE 



AND AFTER THE CLOSURE OF THE CLAMP AT a. 



to beat, while the ventricle remains quiescent. If the ventricle 

 be now stimulated in a rhythmic manner, it may resume rhythmic 

 activity. These facts are taken as an indication that the rhythmic 

 power is developed in unequal degree in the three parts of the 

 heart. 



4. Automaticity. The heart-muscle continuing to contract rhythmi- 



cally, even after removal from the body and without the aid of 

 any external stimulus is said to be automatic in action. This, 

 however, does not exclude the action of an internal stimulus. 



5. Tonicity. The heart-muscle, like the vascular muscle, main- 



tains continuously a certain degree of contraction, upon which 

 the efficiency of the heart as a pumping organ is largely de- 

 pendent. This tone may, however, be increased or decreased 

 by the action of various external agents. Thus the passage of 

 dilute solutions of various drugs e. g., alkalies, digitalis 

 19 



