CIRCULATION. 8? 



vein, which opens into the same auricle at its upper part. 

 These are the vena cavse already mentioned. * 



10. How the Blood is made to Flow. The action of 

 the heart is entirely involuntary. It contracts and dilates 

 with great regularity. Each contraction or beat of the heart 

 forces the blood onward into the arteries. As it again dilates 

 it sucks the blood from the veins into the auricle. While the 

 heart supplies the chief moving power for circulating the 

 blood, the arteries, by virtue of their elastic walls, assist and 

 regulate the flow. With each beat of the heart the arteries 



A B c 



Fia. 28. Circulation of the Blood in the Web of a Frog's Foot, highly 

 magnified. A, an artery; B, capillaries crowded with corpuscles, owing 

 to a rupture just above, where the corpuscles are jammed into an adja- 

 cent mesh ; C, a deeper vein. The black spots are pigment cells. 



expand, to receive the additional volume of blood. These 

 impulses move along the arteries in waves, and can be felt 

 in different parts of the body. They are known as the pulse. 

 The physician usually feels the pulse at the wrist, because it 

 is a convenient place. 



The venous blood flows along in a steady, even stream. 

 The current from behind, pressing it on toward the heart, 

 the squeezing of the veins by the muscles of the body gener- 

 ally, and the suction of the heart, all tend to bring the blood 

 back again. There is, therefore, not much actual pressure in 



