206 



Till INXrRNAI. rNVIRONMFNT OF THE BOOY 



Part III 



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Fig. 12.8. The upper respiratory tract of a child showing half of the ring of 

 lymphatic tissue, the right tonsil, the adenoid, and lingual tonsil, at the back of 

 the throat. All of these are relatively large in children. In mammals some white 

 cells originate in red bone marrow and others in lymphatic tissue. (Courtesy, 

 Clendening: The Human Body. New York, Alfred A. Knopf, Inc., 1930.) 



are to some extent motile and move about in other tissues as well as in 

 the blood, the neutrophils most actively of them all. They are easily ob- 

 served alive in microscopic preparations and when properly warmed, behave 

 like so many active amebas. Neutrophils, lymphocytes, and monocytes wan- 

 der among the cells of the body, rapidly working their way in and out 

 through capillary walls with scarcely any place barred to them (Fig. 12.9). 

 When on its way through a capillary wall, a neutrophil wedges itself between 

 the cells and quickly pushes them apart. Whenever neutrophils reach a place 

 where bacteria are present they at once proceed to engulf them, living up 

 to their name of phagocytes (cell eaters). Within the leucocytes the living 

 bacteria are killed by digestive fluids as live oysters are killed in the human 

 stomach. 



