THE ACTION OF THE HEART 345 



Function of the Auricles. The ventricles have to do the work 

 of pumping the blood through the blood-vessels. Accordingly their 

 walls are far thicker and more muscular than those of the auricles; 

 and the left ventricle, which has to force the blood over the Body 

 generally, is stouter than the right, which has only to send blood 

 around the comparatively short pulmonary circuit. The circu- 

 lation of the blood is in fact maintained by the ventricles, and we 

 have to inquire what is the use of the auricles. Not unfrequently 

 the heart's 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 contract; this contraction 

 merely completing their filling. 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. 



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 disatole 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 into 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 part of 

 the cardiac cycle occupied by the ventricular systole, the auricles 

 have accumulated blood, and when they at last contract they send 

 on into the ventricles this accumulation. Even were the flow from 

 the veins stopped during the auricular contraction this would be of 

 comparatively little consequence, since that event occupies so 

 brief a time. But, although 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. The heart in fact con- 



