378 CIRCULATION AND RESPIRATION 



destroy disease germs that get between the cells, also the 

 germs in the blood. When you receive a wound that 

 penetrates the skin, they form a protecting wall around 

 the wound, and usually keep germs from reaching the 

 remainder of the body. 



When a wound bleeds, it is because a blood vessel has been cut. The 

 blood at once seeks to close the opening by forming a thick mass, called 

 a clot. The clotting, or coagulating, material is fibrin; it is formed out 

 of fibrinogen, one of the proteids of the blood. This substance coagu- 

 lates, even at the blood's temperature, much as the white of egg coagu- 

 lates when heated. If a little absorbent cotton is placed on a wound, 

 its fibers help the fibrin, and the blood is coagulated more quickly. 



379. The Lymph. The lymph is really another form 

 of the blood. It is made out of the plasma and white 

 corpuscles of the blood, which pass through the capillary 

 walls, and out of water and waste materials from the 

 cells. The lymph fills the spaces between the cells. We 

 have learned (cf. 377) that the capillaries give oxygen 

 and food materials to the cells, and that the cells give 

 waste materials to the blood. This is not strictly true: 

 the capillaries give their materials to the lymph, and the 

 cells really get their food supply from the lymph. In 

 the* same way the cells discharge their waste into the 

 lymph, and the lymph passes it on to the capillaries. 

 Thus the capillaries and the cells really exchange material 

 through the lymph that is between them. 



The lymph has no special pumping organ like the heart. For its 

 movement it depends upon the pressure of certain organs of the body, 

 such as muscles. In this respect the movement of lymph is like the 

 movement of blood in certain veins (cf. 376) . The lymph is gradually 



