MECHANISM OF THE CIRCULATION. 



55 



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 per- 

 form the function of 

 a double forcing-pump 

 (Fig 36), so arranged, 

 that the two pistons al- 

 ternate in their move- 

 ments ; thus the liquid 

 chased from the first 

 body of the pump (a) 

 is introduced into the 

 second (b), without being 

 able to retrace its steps, 

 and is thrown by this 

 second pump into the 

 canal (/) representing 

 the arterial system. 



102. When the finger is gently pressed against an 

 artery resting on a firm surface, as a bone, an impulse or jet 

 is felt to strike the finger regularly. This is due to the action 



* a, body of the pump representing the auricle, and receiving the liquid by 

 the caual c ; b, body of the pump representing the ventricle ; d, canal of 

 communication, representing the auriculo-ventricular orifice, furnished with a 

 sucker permitting the fluid to pass from a to b, but opposing its return ; 

 e, the sucker or valve, situated at the opposite orifice of the pump 6, repre- 

 senting the semilunar valves of the aorta, and having the same action as the 

 preceding valve or sucker ; /, canal, representing the aorta. 



Fig. 36.* 



