THE BLOOD 



181 



Water 



Water 



Oxygen 



Sugar etc. 

 BLOOD 

 Protein etc. 



one-celled organisms obtain their supplies directly. The lymph 

 is an internal ocean from which all the cells of the many-celled 

 animal obtain their supplies. 



211. Clotting of blood. When some blood is removed from 

 the blood vessels, whether it is taken out of the body or not, 

 it usually becomes clotted, or thickened. This clotting is 

 brought about by the coagulation, or solidifying, of a certain 

 protein in the blood known as fibrinogen, which means 

 " fibrin-maker." The ferment that causes the coagulation 

 becomes active when the lining of a blood vessel is injured ; 

 or it may be that the 



ferment is formed 

 only at such times. 

 At the mouth of a 

 small cut this clot 

 soon stops the bleed- 

 ing, and furnishes 

 a protective covering 

 until the wound is 

 healed. 



212. Serum. If 

 blood is allowed to 



clot in a glass vessel, we can see the mass of fibers detach 

 themselves from the walls of the vessel, after the threads have 

 shrunk awhile, and the clot floats at last in a clear liquid that 

 is almost colorless or slightly yellowish. This clear liquid is 

 called a serum, and is practically the same as the blood 

 plasma, but lacking the fibrinogen. Whatever is characteristic 

 or distinctive of the plasma of an individual or of a species 

 will be found in the serum. 



213. The white corpuscles. On closer study the white cor- 

 puscles are seen to be cells. In many ways they are like the 

 ameba (see p. 24). They consist of naked protoplasm, have 

 no definite shape, are capable of free movement, are sensitive 

 to chemical and other changes in their surroundings, and are 



LYMPH 



FIG. 68. What goes through the wall of a capillary 



From the blood within the capillary, water, salts, food, 

 and oxygen pass out by osmosis ; from the surrounding 

 lymph, carbon dioxid, urea, and water pass into the blood. 

 White corpuscles work their way through the wall of the 

 capillary, between the cells 



