THE BLOOD-VESSELS AND CIRCULATION 131 



of this network varies considerably in different parts. The walls 

 of capillaries are exceedingly thin, so that fluids very readily ooze 

 through them, and, on this account, the changes which the blood 

 undergoes during its circulation take place chiefly in these vessels. 



After the blood has passed through the meshes of the capil- 

 laries, it is collected up by very small veins. These small veins 

 unite, forming larger and larger veins, till at last they open into 

 the great veins which take the blood direct into the auricles. 



The structure of veins is very similar to that of arteries. 

 Their walls are composed of the same three coats, but they are 

 thinner, and the muscular arid elastic fibres are not nearly so 

 abundant. Hence the walls collapse when the veins are empty. 



Fig. 122. -Capillary Blood- 

 vessels in the Web of a 

 Frog's Foot, as seen with 

 the microscope. 



a, small artery ; b, capillaries ; c, small 

 vein. The arrows show the course 

 of the blood. 



Fig. 123. Diagram showing 

 the Valves of Veins. 



A, part of a vein laid open, with two 

 pairs of valves. 



B, longitudinal section of a vein, 

 showing the valves closed. 



c, portion of a distended vein, ex- 

 hibiting a swelling at a pair of valves. 



Most veins are also provided with valves like the semilunar 

 valves of the heart, sometimes arranged at very short intervals. 

 All these valves are situated with their free edges turned toward 

 the heart. Hence they allow the blood to flow freely towards that 

 organ ; but any backward tendency on the part of the blood 

 would fill these pouch-like valves, causing them to extend across 

 and to close the vein. In small veins the valves are sometimes 

 single, but generally they are arranged in pairs. 



Arteries are generally deeply set, while many of the larger 

 veins are situated very near the surface of the body, so near that 

 their direction, as shown by the bluish tint visible through the 

 skin, may be easily traced. If we lay bare the arm, we notice 

 several of these veins on the front surface, and by pressing the 



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