188 CIRCULATION 



clearlj' the streams of blood plasma and corpuscles in 

 motion. With closer attention we can make out which 

 tubes are arteries, veins, and capillaries. The blood in 

 the arteries flows in spurts or pulses. The capillaries 

 show a steady streaming motion and are so small that the 

 corpuscles move in almost single file through them. The 

 ■\'eins are as large as the arteries but the blood in them 

 flows in a steady stream. If, now, we observe the direc- 

 tion of flow in the several vessels, it will be seen that the 

 blood in the arteries divides and passes into the small 

 capillaries, while from them in turn it passes on into the 

 veins and through these toward the body again. In 

 other words, the capillaries are the connecting links be- 

 tween the arteries and veins, and in passing thi'ough them 

 the blood loses its spurting method of flow and is converted 

 into a steady continuous flow. 



The difference in color between the arteries and veins 

 is explained by the color of the blood in the two tubes. 

 The blood in the arteries is bright red, being full of oxygen, 

 while that in the ^^eins has a darker color owing to the 

 larger proportion of carbon dioxide present. This latter 

 blood is not blue, and that color is due to certain light 

 effects. 



Summary. 1. Arteries convey blood from the heart, and veins 

 return the blood to it 



2. Arteries contain much oxygen and little carbon dioxide, 

 veins contain much carbon dioxide and little oxygen. 



3. Blood in arteries moves in spurts (that is, it pulses), while 

 that in capillaries and veins moves in a steady stream. 



4. Arteries are better protected than veins by being in general 

 placed farther below the surface. (This does not apply to the 

 main vein trimks.) 



