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HUMAN PHYSIOLOGY 



tree, become larger and unite with each other as they approach the 

 heart. The blood supply from the body enters the right heart 

 (auricle) by two large vessels, called, respectively, because of their 

 position, the inferior and superior vence cavce. 



If the wall of a vein is carefully examined, it will be found to 

 be not so thick or so tough as an arter}^ wall. When empty, a 

 vein collapses; the wall of an artery holds its position. If you 

 hold your hand downward for a little time and then examine it, 

 j^ou will find that the veins, which are relatively 

 much nearer the surface than are the arteries, 

 appear to be very much knotted. This appearance 

 is due to the presence of tiny valves inside the veins. 

 These valves open in the direction of the blood cur- 

 rent, but would close if the direction of the blood 

 flow should be reversed (as in case a deep cut sev- 

 ered a vein). As the pressure of blood in the veins 

 is much less than in the arteries, these valves thus 

 aid in keeping the flow of blood in the veins toward 

 the heart. ' 



The Course of the Blood in the Body. — Although 

 the two sides of the heart are separate and distinct 

 from each other, yet ever}^ drop of blood that passes 

 through the left heart likewise passes through the 

 right heart. There are two distinct systems of cir- 

 culation in the body. The 'pulmonary circulation 

 takes the blood through the right auricle and ven- 

 tricle, to the lungs, and passes it back to the 

 left auricle. This is a relatively short circulation, the blood re- 

 ceiving in the lungs its supply of oxygen, and there giving up 

 some of its carbon dioxide. The greater circulation is known as 

 the systemic circulation; in this system, the blood leaves the left 

 ventricle through the great dorsal aorta. A large part of the 

 blood passes directly to the muscles; some of it goes to the ner- 

 vous system, kidneys, skin, and other organs of the body. It 

 gives up its supply of food and oxygen in these tissues, receives 

 the waste products of oxidation while passing through the capil- 

 laries, and returns to the right auricle through the venae cavse. It 



Valves in a 

 vein. 



