The serum is practically the same as the blood plasma, lacking the fibrin- 

 ogen. Whatever is characteristic or distinctive of the plasma of an individual 

 or of a species v^^ill be found in the serum. 



The White Corpuscles There are several types of white blood cor- 

 puscles, all of them resembling the ameba in consisting of naked protoplasm 

 (see page 25). Some of them have no definite shape and move about freely 

 and also eat like the ameba. All seem to be sensitive to chemical changes, 

 and probably other changes, in their surroundings. 



These active corpuscles are very similar in all animals that have blood. 

 Their function has come to be understood only in modern times, chiefly 

 through the work of the Russian biologist Ilya Metchnikoff (1845-1916), 

 who was director of the Pasteur Institute in Paris. 



It helps us to understand the functions of these cells if we recall that 

 whereas the ameba cell carries on all the functions of a living body, the 

 various cells of a many-celled animal, like a butterfly or a baby, are spe- 

 cialists. Now the white corpuscles are in many ways the least specialized 

 cells in the body. They have the general qualities of protoplasm in the 

 greatest degree. They can move, like muscle cells. They are irritable, like 

 nerve cells. They are chemical laboratories, like gland cells. 



As eating cells, white blood corpuscles engulf foreign particles with 

 which they may come in contact. For this reason, Metchnikofl called them 

 phagocytes, that is, "eating cells". They eat and digest the dead particles that 

 result from the breaking down of tissue cells. They may eat also live cells 

 introduced from without, such as bacteria (see page 177). 



As moving cells, the white corpuscles wander about from the lymph to 

 the blood, or vice versa, and even into the intestines. In this way they carry 

 with them dead matter, which is then thrown out. Or they crowd together 

 in large numbers wherever an injury or an invasion by foreign organisms 

 takes place. If an infection is severe, vast numbers of young phagocytes, 

 which originate in the red bone marrow, swarm into the circulating blood. 

 In exceptional conditions die number in the blood increases to three and four 

 times the normal number. From the "blood count" physicians often judge 

 the severity of an infection. 



Pus is formed in a wound by the conflict between the white blood cor- 

 puscles and bacteria. Bacteria destroy some of the corpuscles. Corpuscles 

 liberate a protein-digesting enzyme called trypsin, which digests dead bac- 

 teria and any body cells that may be killed by the bacteria. 



Some of the other white corpuscles appear to take part in the healing of 

 wounds and the repair of injured tissues. These originate in lymphatic 

 tissues. Because of their peculiar behavior in the presence of foreign sub- 

 stances and particles, we have come to think of the white corpuscles as 

 important agents in keeping the body in health. 



188 



