CIRCULATORY SYSTEM 335 



of these disintegrate so rapidly when blood is removed 

 from the body that very little is known of their structure. 

 The plasma of the blood is largely water and is almost 

 colorless. The red color of the blood is due to the red 

 corpuscles which float in it. It contains, however, all the 

 food and waste products which are soluble. This food is 

 carried to all the cells of the body, and the waste products 

 are taken to the excretory organs. The composition of 

 the plasma thus varies greatly in different parts of the 

 body. The plasma diffuses readily through the walls of 

 the blood vessels and thus comes directly in contact with 

 the body cells. As soon as the plasma has left the blood 

 vessels it is called lymph. 



Lymph. Lymph is blood plasma, minus -the red cor- 

 puscles. It is a colorless liquid in which all the tissues 

 are bathed. The watery fluid which collects in a water 

 blister is lymph. From this lymph the body cells take 

 such food substances as they need. In exchange they 

 return to the lymph waste products formed as a result 

 of the action of the cells. Part of this lymph filled 

 with waste products again diffuses directly into the blood. 

 The rest is collected into lymph vessels, which we have 

 learned finally enter the blood vessels. 



Red corpuscles. When a drop of blood is examined 

 under a microscope, the red corpuscles appear as small 

 disk-shaped cells of a reddish yellow color. A corpuscle 

 is round and flat like a coin, but thinner in the middle 

 than near the edge. They are all about the same size, 

 about 1/3,500 of an inch in diameter. They are so 



