CIRCULATORY SYSTEM 343 



bleeding. Evidently the skin is filled with these small 

 capillaries, so small that they can not be seen with the 

 naked eye. The estimated diameter of these vessels 

 is 1/3,000 of an inch. 



If we continue to trace the capillaries, WQ notice that 

 gradually they unite to form larger tubes with thicker 

 walls. These tubes become successively larger and 

 finally form the large veins which enter the heart. 

 The walls of veins are thinner than those of the 

 arteries, but they will not, like arteries, remain dis- 

 tended when there is no blood in them. The blood 

 pressure in veins is low. In order to prevent the 

 backward flow of blood many of the veins are supplied 

 with pocket-like valves. These valves open and thus 

 close the vein if the blood tends to flow back toward 

 the capillaries. 



Lymph vessels. The veins are assisted in returning 

 the blood to the heart by a set of tubes known as the 

 lymph vessels. These vessels originate in the (lymph) 

 intercellular spaces in the tissues. They gradually run 

 together to form larger vessels and finally empty their 

 contents into the veins at the sides of the neck. . 



Questions 



1. State the chief uses of the circulatory system. 



2. What are the three parts of this system? 



3. What is the composition of blood plasma? Its 



use? 



