THE BLOOD AND ITS CIRCULATION 323 



pound microscope, a network of blood vessels will be seen. In 

 some of the larger vessels the corpuscles are moving rapidly and 

 in spurts ; these are arteries. The arteries lead into smaller vessels 

 hardly greater in diameter than the width of a single corpuscle. 

 This network of capillaries may be followed into larger veins in 

 which the blood moves regularly. This illustrates the condition 

 in any tissue of man where the arteries break up into capillaries, 

 and these in turn unite to form veins. 



Structure of the Arteries. - - A distinct difference in structure 

 exists between the arteries and the veins in the human body. The 

 arteries, because of the greater strain received from the blood which 

 is pumped from the heart, have thicker muscular walls, and in 

 addition are very elastic. 



Cause of the Pulse. - The pulse, which can easily be detected by press- 

 ing the large artery in the wrist or the small one in front of and above the 

 external ear, is caused by the gushing of blood through the arteries after 

 each pulsation of the heart. As the large arteries pass away from the 

 heart, the diameter of each individual artery becomes smaller. At the 

 very end of their course, these arteries are so small as to be almost mi- 

 croscopic in size and are very numerous. There are so many that if 

 they were placed together, side by side, their united diameter would be 

 much greater than the diameter of the large artery (aorta) which passes 

 blood from the. left side of the heart. This fact is of very great impor- 

 tance, for the force of the blood as it gushes through the arteries becomes 

 very much less when it reaches the smaller vessels. This gushing move- 

 ment is quite lost when the capillaries are reached, first, because there is 

 so much more space for the blood to fill, and second, because there is 

 considerable friction caused by the very tiny diameter of the capillaries. 



Capillaries. - The capillaries form a network of minute tubes 

 everywhere in the body, but especially near the surface and in the 

 lungs. It is through their walls that the food and oxygen pass 

 to the tissues, and carbon dioxide is given up to the plasma. They 

 form the connection that completes the system of circulation of 

 blood in the body. 



Function and Structure of the Veins. - - If the arteries are supply 

 pipes which convey fluid food to the tissues, then the veins may 

 be likened to drain pipes which carry away waste material from the 



