CIRCULATION 81 



The Lymph. — The walls of the capillaries are so thin 

 that not only do the white corpuscles creep through, but 

 by osmosis the plasma, or liquid portion of the blood 

 with its dissolved substances, passes through. This 

 liquid, now called the lymph, is clear and colorless and 

 contains everything in the blood except red corpuscles, 

 which are too large to pass through the capillaries under 

 ordinary conditions. The lymph bathes the tissues and 

 keeps them moist. It brings the dissolved food mate- 

 rials into such close contact with the tissues that they 

 can without difficulty absorb what they need; and 

 through the activity of the white corpuscles, it removes 

 wastes readily. 



The Lymphatics. — The lymph passes from the capil- 

 laries into spaces between the cells. These spaces com- 

 municate with each other, and into them open the ex- 

 panded ends of small tubes, Called lymphatics. The 

 small lymphatics unite to form larger and larger tubes 

 until two are formed (Fig. 48). These enter the two 

 large veins near the neck, the larger or left one carrying 

 the fats which were taken up by the lacteals, and pour 

 the lymph into the blood through an opening protected 

 by valves which prevent the backward flow of the blood 

 into the lymphatics. The lymph flows because of the 

 pressure caused by muscular activity. There is no heart 

 or pumping organ connected with the lymphatic system 

 in man; in the frog and certain other lower animals, 

 however, where the lymph spaces are very large, four 

 rhythmically contracting organs called lymph hearts 

 force the lymph onward. 



Vaso-Motor Nerves. — Contraction and relaxation of 



