n THE ACTION OF THE VALVES 55 



great arteries, for as we shall see, these arteries are already 

 so full that the blood within them is pressing on their 

 walls with great force. 



It now becomes obvious why the ventricles have so 

 much more to do than the auricles, and why valves are 

 needed between the auricles and ventricles, while none 

 are wanted between the auricles and the veins. 



All that the auricles have to do is to fill the ventricles, 

 which offer no active resistance to that process. Hence 

 the thinness of the walls of the auricles, and hence the 

 needlessness of any auriculo-venous valve, the resistance 

 on the side of the ventricle being so insignificant that it 

 gives way, at once, before the pressure of the blood in the 

 veins. 



On the other hand, the ventricles have to overcome a 

 great resistance in order to force fluid into elastic tubes' 

 which ai'e already full ; and if there were no auriculo- 

 ventricular valves, the fluid in the ventricles would meet 

 with less obstacle in pushing its way backward into the 

 auricles and thence into the veins, than in separating the 

 semilunar valves. Hence the necessity, firstly, of the 

 auriculo-ventricular valves ; and, secondly, of the thick- 

 ness and strength of the walls of the ventricles. And 

 since the aorta, systemic arteries, capillaries, and veins 

 form a system of tubes, which, from a variety of causes, 

 offer more resistance than do the pulmonary arteries, 

 capillaries, and veins, it folhjws that the left ventricle 

 needs a thicker muscular wall than the right. 



Thus, at every systole of the auricles, the ventricles 

 are filled and the auricles emptied, the latter being slowly 

 refilled by the pressure of the fluid in the great veins, 

 which is amply sufticient to overcome the passive resist- 

 ance of the relaxed auricular walls. And, at every systole 

 of the ventricles, the arterial systems of the body and 

 lungs receive the contents of these ventricles, and the 

 emptied ventricles remain ready to be filled by the 

 auricles. 



