2IO 



How a Co?;iplex An'wial Uses Food unit iv 



spleen contracts more vigorously than 

 usual and thus increases the number of 

 red cells in circulation. This is interesting 

 because in both cases it is an advantage 

 to the person to have more corpuscles. 

 When exercising he needs more oxygen. 

 At high altitudes there is less oxygen in 

 the air and a large number of corpuscles 

 is desirable. 



White corpuscles. We have many kinds 

 of white corpuscles, or leucocytes (lew'- 

 ko-sites). Those that are most numerous 

 are large cells that resemble an ameba in 

 shape; that is, they have no definite shape, 

 since their soft protoplasm streams now 

 in one direction, now in another, form- 

 ing pseudopods. Their protoplasm is 

 quite granular and the nucleus is large 

 and usually shaped like an irregular club. 

 These white corpuscles move about much 

 as an ameba would. The great Russian 

 biologist Eli Metchnikoff (i 845-1 91 6) 

 discovered these cells near the end of the 

 last century and called them phagocytes 

 (fag'o-sites). They can push their way 

 between the cells that make up the walls 

 of the capillaries and get in among the 

 tissue cells. Here they engulf and grad- 

 ually digest bacteria or any other parti- 

 cles that are present. They serve as tiny 

 scavengers (eaters of unwanted sub- 

 stances) in the body. When bacteria 

 enter the body, millions of the phago- 

 cytes and some of the other kinds of 

 white cells are soon attracted to the spot. 

 The other kinds are helpful in surround- 

 ing this whole region and keeping it 

 separated from the neighboring tissues. 

 The large white corpuscles begin at once 

 to devour bacteria. As many as twenty 

 bacteria have been found within one 

 corpuscle. Often the white corpuscles 



White cell fphagocyfej 



Bacteria 



Fig. 211 Three white cells (phagocytes) de- 

 stroying bacteria by engulfing and digesting 

 them. How is this activity of the white cells 

 of benefit to the body? 



are killed by the poisons secreted by the 

 bacteria. The dead bodies of the white 

 corpuscles together with destroyed tis- 

 sues is pus. This whole region, or abscess 

 (ab'sess), is red, swollen, and hot to the 

 touch. Much blood is present. 



Some kinds of white corpuscles are, 

 like the red corpuscles, made in the red 

 marrow of the bones. Other kinds of 

 white corpuscles are made elsewhere, in 

 what we call lymph glands. You will 

 read of this later. 



Blood platelets and clotting. The third 

 kind of blood cell, the platelet (small 

 plate) is the smallest. It has no nucleus. 

 Platelets are connected in some way 

 with the clotting of the blood. You have 

 seen how the blood which oozes out of 

 a small cut hardens or clots. If it did not 

 clot and thus plug up the blood vessel, 

 the blood would keep right on flowing. 



According to one theory of clotting, 

 the platelets together with other cells 

 start the process of clotting by breaking 

 up when the blood vessel is damaged. 

 As they break up they release a sub- 

 stance. This substance indirectly causes 



