THE BLOOD AND CIRCULATION 267 



and the various organs into the blood vessels. It is 

 replenished by fluid which diffuses from the finer blood 

 vessels into the surrounding tissues. One of the chief 

 functions of lymph vessels is the absorption and transfer 

 of materials into the blood system. 



Closely associated with the lymph vessels are the lymph 

 glands. Some of these may be felt in the groins or under 

 the jaw. They are centers in which leucocytes arise, and 

 they often become enlarged in disease. Infections often 

 travel from their point of origin, along the course of the 

 lymph vessels and frequently cause swelling and suppura- 

 tion of the neighboring lymph glands. The tonsils contain 

 much lymphatic tissue they are very apt to become in- 

 fected and often have to be removed because the in- 

 fection may spread from these organs and produce a 

 variety of ill effects. 



The organs of circulation are to a certain extent under 

 the control of the nervous system. The peripheral arteries 

 may contract upon nervous stimulation, as when one sud- 

 denly grows pale, or they may become relaxed and filled 

 with blood under other circumstances, as in blushing. 

 The exercise of a part causes it to receive an increased 

 blood supply, and after a meal there is an unusually large 

 amount of blood sent to the organs of digestion. Hard 

 thinking brings an increased blood flow to the brain. 

 Serious mental work after a heavy meal when the blood 

 is occupied with the business of digestion is therefore an 

 uphill task, as doubtless most of you have already found 

 out. 



The proper working of the organs of circulation is very 

 important for the maintenance of physical vigor. The 

 heart may be weakened by over exercise, but moderate 

 exercise strengthens it as well as gives tone to the blood 

 vessels. The heart is more frequently injured by bad 



