CIRCULATION OF THE BLOOD 



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fraction of an inch in length. They combine and form 

 larger vessels known as veins. In all parts of the body 

 may be found small veins which unite to form larger 

 and still larger ones, until there are only two great 

 trunks, the ascending and descending vena caves, that 

 empty into the right auricle. The function of the veins 

 is to return the blood from the capillaries to the heart. 

 The veins differ from the arteries in that their walls are 



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FIG. 67. Showing valves in veins (Zuppke). 



lighter and less elastic, and in that they are provided with 

 valves which do not obstruct the flow of blood toward 

 the heart but prevent its return to the cells. They lie 

 nearer the surface of the body than the arteries and 

 form the purple-looking streaks which we frequently 

 see on the hands and arms of a thin person. Blood 

 containing considerable oxygen is brig/it red in color; but 

 as it returns from the cells, where it has exchanged most 

 of its oxygen for carbon dioxid and other waste 

 materials, it is very dark red. The purple color is due to 



