THE CIRCULATION 



trivances both for holding the blood and for keeping it in 

 motion through the body. The heart, which is the chief 

 organ for propelling the blood, acts as a force pump, 

 while the arteries and veins serve as tubes for conveying 

 the blood from place to place. Moreover, the blood vessels 

 are so connected that the blood moves through them in a 

 regular order, performing two well-defined circuits. 



The Heart. The human heart, roughly speaking, is 

 about the size of the clenched fist of the individual owner. 

 It is situated very near 

 the center of the tho- 

 racic cavity and is almost 

 completely surrounded 

 by the lungs. It is cone- 

 shaped and is so sus- 

 pended that the small 

 end hangs downward, 

 forward, and a little to 

 the left. When from ex- 

 citement, or other cause, 

 one becomes conscious 

 of the movements of the 

 heart, these appear to 

 be in the left portion of 

 the chest, a fact which FIG. 13. Heart in position in thoracic 



cavity. Dotted lines show position of dia- 

 accounts for the errone- ph ragm and of margins of lungs. 



ous impression that the 



heart is on the left side. The position of the heart in 



the cavity of the chest is shown in Fig. 13. 



The Pericardium. Surrounding the heart is a protec- 

 tive covering, called the pericardium. This consists of a 

 closed membranous sac so arranged as to form a double 

 covering around the heart. The heart does not lie inside 



