THE BLOOD AND ITS CIRCULATION 



61 



it. They also surround and destroy poisons which happen 

 to get among the cells. We will speak of this later when 

 we talk of inflammation and of catching cold. (See 137.) 



112. Plasma. The liquid part of the blood is called 

 plasma. It is water, in which a little albumin and mineral 

 matters are dissolved. The albumin and the mineral mat- 

 ters are the real food of the cells. 



When blood is drawn from the body, it soon becomes 

 like jelly, or it clots, as we say. In clotting, some of the 

 .albumin of the plasma 

 becomes hard like a boiled 

 egg and makes the whole 

 blood like jelly. Soon 

 the clot begins to grow 

 smaller and to squeeze 

 out the liquid parts of 

 the blood. Then the 

 clot will float in a straw- 

 colored liquid called 

 serum. In a cut a clot 

 reaches into the mouth 

 of each blood tube and 

 stops the opening, so 

 that no more blood can 

 ing is stopped. 



113. Arteries. The blood is carried to the cells of the 

 body through tubes which branch and divide again and 

 again, until they reach every part of the body. The tubes 

 which carry blood to the cells of the body are called 

 arteries. Their sides are strong and tight, so that they 

 cannot burst or leak. In them are muscles which can 

 make them larger or smaller, so as to let the right amount 

 of blood pass through for the use of the cells of the body. 



Arrangement of capillaries. 



a smallest artery. 



b smallest vein. 



c network of capillaries. 



flow. This is the way all bleed- 



