THE HUMAN BODY. 



Fig. 34. 

 CAPILLARY PLEXUS MAGNIFIED. 



there are so many of them, that their total blood-channel 

 is really wider. It is estimated that it is three hundred 

 times as wide as the aorta. 



7. Though many men of great minds gave much study 

 to the human body, it was not learned until less than 

 three hundred years ago that the blood circulated. Wil- 

 liam Harvey, an English physician, made the discovery, 



and gave it to the world. 



8, The walls of the capillaries 

 are a single layer of a thin mem- 

 brane. Through them nutritious 

 fluids soak out, and waste fluids 

 soak in. 



9, Some points in which ar- 

 teries and veins differ : 



1. In the direction of their cur- 

 rent. In arteries it always runs from the heart : in veins 

 it always runs toward the heart. 



2. In position. We do not find 

 large arteries running just beneath 

 the skin, as veins do. 



s. In color. The blue tint of their 

 blood shows through the walls of 

 the veins. 



4. In thickness. The arterial walls 

 are thicker and firmer. 



5. The veins have valves at inter- 

 vals in their course, which allow the 

 blood to move forward, but not 

 backward. 



10, In case of a wound, we can 

 tell whether the blood is from an artery or a vein 



Fig. 35. 



VEIN LAID OVEN, SHOWING 

 THE VALVES. 



