CHAPTER VII 

 THE CIRCULATION OF THE BLOOD 



187. The Use of Blood. Every child knows that if he 

 clits his finger, or even pricks it with a needle, the blood 

 will flow. What is true of the finger is also true of every 

 other part of the body, except the outermost layer of the 

 skin, the hair, and the nails. 



The tiny blood vessels go everywhere, through the mus- 

 cles and nerves, over and within the brain, through every 

 part of every bone, the blood flows everywhere. Wher- 

 ever it goes, the blood has something to bring to the tissues 

 and something to carry away. 



The blood, in its ceaseless round, not only brings new 

 material for repair, but it also becomes a kind of sewer 

 stream that carries waste matters to organs whose duty it 

 is to cast them out of the body. 



188. Properties of Blood. The blood is a red, somewhat 

 sticky fluid, thicker than water. It has a peculiar smell 

 and a saltish taste. If we look at a small drop of freshly 

 drawn blood under a microscope, we shall see that it is not 

 so simple as it looks. It consists of fluid of a light straw 

 color, called the plasma, and an enormous number of little 

 bodies called corpuscles, floating in the liquid. These cor- 

 puscles are of two kinds, red and white. 



189. Why the Blood looks Red. The red color of the 

 blood is due to millions of little red corpuscles which float 



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