314 CIRCULATORY SYSTEM. 



into the auricles that is, the venae cavae and pulmonary veins 

 are muscular fibers ; these fibers contract, diminishing the size of 

 the orifices of the veins, thus taking the place of valves, and par- 

 tially preventing a back-flow of blood into these vessels. Then 

 the muscular fibers of the auricles in contiguity with these fibers 

 contract, the movement spreading to the adjoining fibers until the 

 wave of contraction has reached the ventricles. This auricular 

 contraction forces more blood into the ventricles, and as the fibers 

 relax the blood enters the auricles again from the veins. It will 

 thus be seen that the interval of time during which the venous 

 flow is arrested is the briefest possible. The principal office of the 

 auricles is to serve as reservoirs to supply the ventricles; the 

 work they do in completing the filling of these cavities is com- 

 paratively unimportant. 



The auricular systole is followed by the systole of the ventri- 

 cles. These cavities are at this time filled with blood, and the 

 auriculoventricular valves are nearly closed, the segments having 

 been raised up by the blood from the auricles. The ventricles, as 

 has been stated, contract en masse, and the blood which they con- 

 tain is compressed with great force. Under the pressure it tends 

 to escape from the ventricles through all outlets on the right side 

 through the right auriculoventricular orifice back into the right 

 auricle, and through the pulmonary orifice into the pulmonary 

 artery ; on the left side through the left auriculoventricular orifice 

 into the left auricle, and through the aortic orifice into the aorta. 

 The pressure of the blood instantly closes the tricuspid valve, and 

 thus prevents the blood from going back into the right auricle. 

 The same force closes the mitral valve, and regurgitation of 

 blood into the left auricle is made impossible. The pulmonary 

 and aortic valves, as has been stated, open from the ventricles into 

 the arteries. At the beginning of the ventricular systole these valves 

 are closed, but when systole occurs the pressure of the blood forces 

 them open, and the contents of the ventricles, 70 c.c. for each, are 

 propelled into the pulmonary artery and the aorta respectively. 

 Authorities differ as to the amount of blood which is expelled from 

 the ventricle at each pulsation, and which is termed the pulse 

 volume ; some place it as low as 50 c.c., and others as high as 190 

 c.c. Stewart gives it as his opinion that the average amount of 

 blood thrown out by each ventricle at each beat is not more than 

 70 c.c. or 80 c.c. (87 grams). This agrees closely with Tiger- 

 stedt's calculation, which places the amount at 69 c.c. If the 

 average amount is 70 c.c., the whole blood of the body would pass 

 through the heart in about a minute. In accomplishing this the 

 ventricles have to overcome the pressure which the blood already 

 in the arteries is exerting on the other side of the valves to keep 

 them closed. This pressure in the arteries is equal to a column 

 of mercury, approximately, 150 mm. high, and in the pulmonary 



