CIRCULATION OF THE BLOOD 3 



differ from, the arteries. They resemble the arteries in 

 conveying the blood and in having muscular walls. 

 They also resemble the arteries in their general dis- 

 tribution, for most arteries have a vein accompanying 

 them. One difference between the veins and the arteries 

 is that the veins tend to be larger. Moreover, the veins 

 have much thinner and less muscular walls. They 

 are therefore less distended by the blood that enters them 

 from the capillaries than are the arteries by the blood that 

 enters from the heart. As a result, the pressure in the 

 veins is lower than in the arteries or even than in the 

 capillaries. Blood, therefore, normally flows from the 

 capillaries into the veins. 



If the veins into which we have traced the capillaries 

 be followed farther, we should find that they join or unite 

 into larger veins. These larger veins in their turn join 

 still larger veins. Finally, we should trace the veins to 

 one very large vein, the so-called vena cava, which enters 

 the heart. 



We may now consider certain other differences between 

 the arteries and veins. Firstly, we may note that the 

 veins differ from the arteries in that they are equipped 

 here and there, at irregular intervals throughout their 

 course, with valves (Plate VII. Fig. 5). We have already 

 seen that where the aorta arises from the heart there is a 

 set of valves which prevents the blood from returning 

 to the heart. Somewhat similar though less efficient 

 valves are scattered through the course of the veins. 

 The cusps of these valves in the veins are so arranged 

 that they direct the flow of blood away from the capillaries 

 and towards the heart. 



Another and very important difference between arteries 

 and veins is in the character of the blood that they con- 

 tain. The blood in the arteries is bright red, and that 

 in the veins is dark red. Blood enters the capillaries 



