FUNCTION OF THE AURICLES. 225 



tract 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. For at that mo- 

 ment 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 is never so great as to 

 obliterate their cavities; while the ventricles contain no 

 blood at the end of their systole. 



The auricles also govern to a certain extent the amount 

 of work done by the ventricles. These latter contract with 

 more than sufficient force to completely drive out all the 

 blood contained in them. If the auricles contract more 

 powerf ully and empty themselves more completely at any 

 given time, the ventricles will contain more blood at the 

 commencement of their systole, and have pumped out more 

 ut its end. "Now as we shall see in Chapter XVII., the 

 contraction of the auricles is under the control of the 

 nervous system; and through the auricles the whole work 

 of the heart. In fact the ventricles represent the brute 

 force concerned in maintaining the circulation, while 

 the auricles are part of a highly developed co-ordinating 

 mechanism, by which the rate of the circulation is governed 

 according to the needs of the whole Body at the time. 



The Work Done by the Heart. This can be calculated 

 with approximate correctness. At each systole each ven- 

 tricle sends out the same quantity of blood about 180 

 grams (6.3 ounces); the pressure exerted by the blood 

 in the aorta against the semilunar valves and which the 

 ventricle has to overcome is about that which would be ex- 

 eried on the same surface by a column of mercury 200 



