THE HEART 253 



is a particular spot in. the right auricle (the sino-auricular 

 node) from which the process radiates. For the purposes 

 of elementary description we may say that the two auricles 

 contract simultaneously. The systole of the auricles is 

 brief and not at all forcible. Its effect is to transfer a 

 small quantity of blood to the ventricles which are thus 

 put upon a stretch beyond their previous state. As the 

 auricles relax after their momentary contraction, the 

 blood recoiling from the tense walls of the ventricles 

 brings together the flaps of the mitral and tricuspid 

 valves. 



It is to be noted that there are no valves to guard the 

 openings by which the veins empty into the auricles. 

 Hence it may be asked why the contraction of the auricles 

 should not turn back the blood in these vessels. As a 

 matter of fact there is something of a check imposed on 

 the flow in the veins of the thorax and neck when the 

 auricles are contracting. But it is only slight because 

 the time of the backward thrust is so short and the 

 momentum of the blood column so great. It is probable 

 that the systole of the auricles has a progressive or 

 peristaltic character, tending to push the blood toward 

 the ventricles rather than in the reverse direction. 



When the mitral and tricuspid valves have been 

 closed there is a brief pause. It is an interval during 

 which the excitation is being transmitted from the 

 tissue of the auricles to that of the ventricles. There is 

 a zone between the upper and the lower chambers of the 

 heart in which there is little or no muscle. But a peculiar 

 conducting strand, the bundle of His, bridges this 

 region and maintains the physiologic continuity of auricle 

 and ventricle. Through the bundle of His the muscle 

 of the ventricles is presently stimulated and the systole 

 of these chambers follows. This is the essential feature 

 of the heart-beat. 



The two ventricles are contracted at the same time. 

 Their systole occupies about 0.3 second under aver- 

 age conditions. The pressure developed quickly rises 



