232 THE HUMAN BODY. 



action is described as if the auricles first filled with blood and 

 then contracted and filled the ventricles; and then the latter 

 contracted and drove the blood into the arteries. From the 

 account given above, however, it will be seen that the events 

 are not accurately so represented, but that during all the 

 pause blood flows on through the auricles into the ventricles, 

 which latter are already nearly full when the auricles con- 

 tract; this contraction merely completing their filling and 

 finishing the closure of the auriculo-ventricular valves. The 

 real use of the auricles is to afford a reservoir into which 

 the veins may empty while the comparatively long-lasting 

 ventricular contraction is taking place: they also largely 

 control the amount of work done by the heart. 



If the heart consisted of the ventricles only, with valves 

 at the points of entry and exit of the blood, the circulation 

 could be maintained. During diastolo the ventricle would 

 fill from the veins, and during systole empty into the arteries. 

 But in order to accomplish this, during the systole the valves 

 at the point of entry must be closed, or the ventricle would 

 empty itself kito the veins as well as into the arteries; and 

 this closure would necessitate a great loss of time which 

 might be utilized for feeding the pump. This is avoided by 

 the auricles, which are really reservoirs at the end of the 

 venous system, collecting blood when the ventricular pump is 

 at work. When the ventricles relax, the blood entering the 

 auricles flows on into them: but previously, during the -ffo 

 of the cardiac cycle occupied by the ventricular systole, the 

 auricles have accumulated blood, and when they at last con- 

 tract they send on into the ventricles this accumulation. 

 Even were the flow from the veins stopped during the auric- 

 ular contraction this would be of comparatively little conse- 

 quence, since that event occupies so brief a time. But, al- 

 though no doubt somewhat lessened, the emptying of the 

 veins into the heart does not seem to be, in health, stopped 

 while the auricle is contracting. For at that moment the 

 ventricle is relaxing and receives the blood from the auricles 

 under a less pressure than it enters the latter from the veins. 

 The heart in fact consists of a couple of " feed-pumps " the 

 auricles and a couple of "force-pumps" the ventricles; 

 and so wonderfully perfect is the mechanism that the supply 

 to the feed-pumps is never stopped. The auricles are never 

 empty, being supplied all the time of their contraction, which 



