1 10 THE CIRCULATION. 



and is thence propagated towards the ariculo- ventricular 

 opening ; but the last part which contracts is the aricular 

 appendix. The effect of this contraction of the auricles is 

 to propel nearly the whole of their blood into the ventricles. 

 The reflux of blood into the great veins is resisted not only 

 by the mass of blood in the veins and the force with which 

 it streams into the auricles, but also by the simultaneous 

 contraction of the muscular coats with which the large 

 veins are provided for some distance before their entrance 

 into the auricles ; a resistance which, however, is not so 

 complete but that a small quantity of blood does regurgi-^ 

 tate, i.e., flow backwards into the veins, at each auricular 

 contraction. The effect of this regurgitation from the 

 right auricle is limited by the valves at the junction of the 

 subclavian and internal jugular veins, beyond which the 

 blood cannot move backwards ; and the coronary vein, or 

 vein which brings back to the right auricle the blood 

 which has circulated in the substance of the heart, is pre- 

 served from it by a valve at its mouth. 



The blood which is thus driven, by the contraction of the 

 auricles, into the corresponding ventricles, being added to 

 that which had already flowed into them during the heart's 

 pause, is sufficient to complete the dilatation or diastole of 

 the ventricles. Thus distended, they immediately contract : 

 so immediately, indeed, that their contraction, or systole, 

 looks as if it were continuous with that of the auricles. 

 This has been graphically described by Harvey in the 

 following passage : " These two motions, one of the ven- 

 tricles, another of the auricles, take place consecutively, 

 but in such a manner that there is a kind of harmony, or 

 rhythm, present between them, the two concurring in such 

 wise that but one motion is apparent ; especially in the 

 warmer blooded animals, in which the movements in ques- 

 tion are rapid. Nor is this for any other reason than it is 

 in a piece of machinery, in which, though one wheel gives 

 motion to another, yet all the wheels seem to move simul- 



