LYMPH GLANDS 151 



masses of small cells, the function of which, like that of 

 the white blood corpuscles, is to destroy microbes and 

 other foreign substances. They are so constructed that no 

 portion of the lymph can escape passing between a large 

 number of their cells and being exposed to their influence 

 before it continues on its passage towards the veins. If 

 microbes are present in overwhelming numbers, the cells 

 of the first protecting lymph gland may not be able to 

 kill them all or even prevent their development, which 

 then takes place in the overcome gland itself. In this 

 case, the lymph glands next in order in the course block 

 the progress of the microbes and continue to the limit 

 of their ability the good work of exterminating them. 

 In extreme cases,' however, an invasion of microbes may 

 overcome gland after gland in the course of the lym- 

 phatics until finally they enter the blood stream and are 

 scattered to all parts of the body. The result is a general 

 infection, or blood poisoning, which always constitutes a 

 grave menace to life. In this way, an infected scratch or 

 cut may eventually have serious results. 



Control of blood supply to tissues. In order that 

 the tissues may receive as much food and oxygen as they 

 need when active and yet the total amount of the blood 

 be kept as small as possible, the blood is sent to them 

 copiously during activity and sparingly during rest. 

 To accomplish this, a definite and accurate control of 

 the distribution of the blood is required. Certain of the 

 nerves are sensitive to the activity of the tissues and 

 control the muscle fibres in the walls of their arteries. 

 When tissue cells, because of their activity, require more 

 food and oxygen, the nerves cause the muscle fibres in 

 the walls of their arterioles to relax, thus allowing more 

 blood to pass through them. When, on the contrary, 

 the tissues become quiet and so require less food and 



