WORK DONE BY THE HEART. 221 



quently the heart's action is described as if the auricles 

 first filled with blood, and then contracted and filled the 

 ventricles; and then the ventricles contracted and drove the 

 blood into the arteries. Erom the account given above, 

 however, it will be seen that the events are not accurately 

 so represented, but that during all the pause the blood 

 flows on through the auricles into the ventricles, which 

 latter are already nearly full when the auricles contract; 

 this contraction merely completes the filling of the ventri- 

 cles, and finishes the closure of the auriculo-ventricular 

 valves. The main 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. 



The Work done daily by the Heart. At each beat each 

 ventricle pumps on rather more than six ounces (say four- 

 teen tablespoonfuls) of blood. The elastic aorta and the 

 pulmonary artery are full, and resist the pumping of more 

 liquid into them, just as an elastic bag filled with water 

 could only have more sent into it by force; to get more in 

 one would have to stretch the bag more. The resistance 

 opposed by these arteries to receiving blood from the heart 

 has been measured in some of the lower animals, and calcu- 

 lations made from them to man. According to these the 

 work which the left ventricle does every day, sending 6J 

 ounces of blood seventy times a minute into the aorta, is 

 enough to lift one pound t S$5,584 feet high; and the work 

 done by the right ventricle would lift one pound 108,528 



What parts of the heart does the blood enter during the pause? 

 What is the condition of the ventricles as regards fullness at the 

 end of the pause? What is done by the auricular contraction? 

 What is the chief use of the auricles? 



How much blood does each ventricle pump out at every beat? 

 What resists the ventricular emptying? Illustrate. How much 

 work does a man's left ventricle do daily? How much the right? 



