CIRCULATION 411 



When the blood is examined, it is found to consist of a 

 watery liquid, called the plasma, a great number of little 

 disk-shaped bodies, the red corpuscles, and some irregular 

 whitish bodies, the white corpuscles (Figure 132). 



The white corpuscles are protoplasmic cells possessing 

 the power of movement and even of working their way out 

 of the blood vessels. They are the soldiers of defense of 

 the human body. When a white corpuscle comes in contact 

 with a disease germ, the body of the corpuscle takes the 

 germ into it and tries to digest it. The germ in turn tries 

 to multiply inside the corpuscle and to feed on it. Unless 

 the germs increase in number too rapidly, 

 the white corpuscles come off victorious. 

 The blood also provides other substances 

 that are probably even more important than 

 white corpuscles in fighting disease. Some 

 of these substances kill disease germs and A WHITE CORPUS- 

 others counteract germ poisons. CLE DIGESTING A 



_, . , / - GERM (Greatly 



The mam function of the red corpuscles magnified.) 

 is to carry oxygen from the lungs to the 

 different living cells of the body. They contain a pigment, 

 hcemoglobin, which carries the oxygen and gives the blood 

 its color. The plasma, an exceedingly complex fluid, is 

 composed largely of water, but contains the nutrient and 

 waste materials supplied by the different organs of the body. 



The blood passes through different kinds of vessels. 

 Those leading from the heart are called arteries, and those 

 returning to the heart are called wins. As the arteries 

 proceed from the heart they divide continually, becoming 

 smaller and smaller until they terminate in very small, 

 thin- walled vessels called capillaries. These capillaries 

 unite and form veins. Thus the blood is continually flow- 



