THE CIRCULATION OF THE BLOOD 



139 



195. General Plan of Circulation. Since all the tissues 

 stand in such constant need of blood, there must be some 

 special machinery by which this "river of life " is made to 

 keep up its constant flow through every nook of the body. 

 Let us now begin the 



study of this apparatus. 



In the first place, 

 there is in the chest a 

 little pump called the 

 heart, from which tubes 

 are distributed to all 

 parts of the body. 



One set of tubes, 

 called arteries, carries 

 the blood from the 

 heart. 



Another set of tubes, 

 called veins, brings the 

 blood back to the heart. 



This continued flow 

 of blood through the 

 body, to and from the 

 heart, is called the cir- 

 culation of the blood. 



196. The Circulation 

 compared to the Water 

 Service of a City. The 

 way in which blood is 



made to flow through vessels of the body may be compared in 

 a general way to the manner in which water is supplied to a 

 city. The heart is the pumping engine which forces the blood 

 into the main pipes for the supply of the several districts. 



FIG. 85. Anterior View of the Heart. 



A, superior vena cava ; B, right auricle ; C, right 

 ventricle ; Z), left ventricle ; E, left auricle ; 

 F, pulmonary vein ; H, pulmonary artery ; 

 K, aorta ; L, right subclavian artery ; M, right 

 common carotid artery ; N, left common carotid 

 artery. 



