MECHANISM OF THE CIECULATION. 59 



the semilunar, permitting the blood to flow freely into the 

 artery but not to return into the ventricle ; distributed by 

 the aorta and its branches to every part of the body and to 

 the fleshy walls of the heart itself, the blood is taken up by 

 the venous capillaries, and returned to the right auricle by 

 the two venae cavse and coronary vein; collected in this 

 auricle it is strictly venous blood, but has already received 

 the product of digestion. From this auricle, and by its 

 action, the blood is driven through the right auriculo-ventri- 

 cular orifice into the right ventricle ; the tricuspid valve per- 

 mits it to pass into the ventricle, but not to return. By the 

 action of this ventricle the blood is forced into the pulmonary 

 artery, and by it to the lungs, thence to be returned (aerated) 

 to the left auricle by means of the pulmonary veins. At the 

 entrance of the pulmonary artery there are, as in the aorta, 

 three semilunar valves, permitting the blood to pass freely 

 into the artery, but not to return. From the mechanism 

 alone of the valves alluded to, the circulation of the blood 

 might easily have been foretold. 



100. Course of the Blood in the Arteries. Contrary 

 to what might have been expected, the blood flows in the 

 arteries in a continued stream, and with considerable force. 

 This is due chiefly to the action of the heart itself, but partly 

 also to the elasticity of the arteries themselves. The influence 

 of the elasticity of the walls of the arteries on the passage of 

 the blood is proved by placing two ligatures, at a certain 

 distance from each other, on a large artery in a living animal, 

 and then puncturing the vessel at any point between the 

 ligatures. The blood contained in this insulated portion of 

 the vessel is thrown out of it with considerable force. Thus, 

 by means of the elasticity of the arteries, the jet of blood, or 

 the intermittent movement impressed on the blood by the 

 action of the heart, is transformed into a continuous' flow or 

 stream. In the capillaries, it is presumed that the blood 

 flows on by this means alone ; but some suppose them to be 

 muscular. 



101. Thus the left cavities of the heart perform the 

 function of a double forcing-pump (Fig. 45), so arranged, 

 that the two pistons alternate in their movements; thus 

 the liquid chased from the first body of the pump (a) is 

 introduced into the second (5), without being able to retrace 

 its steps, and is thrown by this second pump into the canal 

 (f) representing the arterial system. 



