CHAPTER III 

 THE CIRCULATORY OR VASCULAR SYSTEM 



THE organs of the circulatory or vascular system are 

 the heart, the arteries, the veins, and all structures con- 

 cerned in the propulsion or onward movement of the blood. 



Circulation of the Blood. The heart is the central 

 station to which the blood is constantly returned, and from 

 which it is as constantly distributed to all parts of the 

 body. In making one complete revolution of the body the 

 blood passes twice through this organ, making its circuit 

 in the following manner: 



As it returns from all parts of the system, the blood is 

 emptied into the right auricle of the heart by what is 

 termed the ascending and the descending vena cava; thence 

 it passes into the right ventricle, the contraction of the 

 heart propelling it through the pulmonary artery into the 

 lungs. From this point the blood, oxygenated by its 

 passage through the lungs, again returns through the four 

 pulmonary veins (this is the only instance in the adult 

 circulation where the veins carry arterial blood) into the 

 left auricle, passing thence into the left ventricle; from 

 which, by the contraction of the heart, it is propelled 

 through the aorta and its numerous branches and dis- 

 tributed to every part of the body. 



THE HEART 



The heart is a hollow, muscular, pear-shaped organ, 

 placed obliquely between the lungs. It is nearer the 



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