26 CIRCULATION OF THE BLOOfi. 



It is this which we feel when we place our hand 

 over the fifth and sixth ribs. 



The heart is divided into four cavities, or cham- 

 bers, which have thick walls of muscle, and it is 

 into these chambers that the blood is received, and 

 by these muscles that it is forcibly pushed out. 

 Besides these, it has several very curious valves to 

 keep the blood flowing in its proper directions. 

 The heart may be called double, one part being 

 employed in forcing the blood through the body 

 at large, and another in forcing it through the 

 lungs. 



We have mentioned how our food is changed 

 into blood ; let us now see in what way the blood 

 is passed to all parts of the system, for the purpose 

 of nourishing them, and supplying the waste. 



The heart is the great centre of the circulation, 

 and is placed between the two sets of blood vessels 

 already described ; namely, the arteries and veins. 

 From these last, one of the chambers of the heart 

 is filled with dark or venous blood, no longer fitted 

 to serve the wants of the system, as it is mixed 

 with the chyle and other matters. This chamber 

 contracts, and forces it into a second and stronger 

 one, which, in its turn, drives it through the lungs. 



.Here it gives off its impurities, and becomes a 

 bright red, or arterial blood. From hence it 

 flows back into a third chamber, and from this into 

 the fourth, which is the most powerful of them all, 

 and from which the great artery arises. Into this 



