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HUMAN PHYSIOLOGY 



Trace the artery until it breaks out into very tiny tubes, the capillaries. 

 Notice that in the capillaries the diameter of the tube is little more than that 

 of a red corpuscle. Follow the capillaries until they come together to form 

 a vein. What difference in the movement of the blood do you notice in the 

 veins? 



Capillary circulation in the web of a frog's foot, as seen imder the compound microscope; 

 a, b, small veins; d, capillaries in which the oval corpuscles are seen to follow one anothei 

 in single series; c, pigment cells in the skin. 



jStructure 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. In the earthworm, we found that certain parts of 

 the blood vessels take up the work of pumping the blood. These vessels 

 which connect the dorsal with the ventral blood vessels are called hearts. 

 Each is a single muscular tube. The fish has such a heart. In the higher 

 vertebrates the heart is more complex, being composed of two such muscu- 



