42 



HUMAN BIOLOGY 



These cells are therefore too small to be seen by the most 

 powerful microscopes. 



The life history and method of transmission of the micro- 

 scopic animal that causes malaria has already been dis- 

 cussed in connection with the study of the Anopheles 

 mosquito (A. B., 40). Likewise, it has been demonstrated 

 beyond a cavil that infection from a yellow-fever patient can 

 only be brought about through the agency of the Stegomyia 

 mosquito. Hence, to eradicate these diseases entirely, we 

 need only to exterminate all Anopheles and Stegomyia mos- 

 quitoes . Sleeping sickness is a dread 

 disease of the tropics which is due 

 to a kind of Protozoan something 

 like a paramecium. 



Ill ---cilia 



mucous cells in various stages of 

 secretion 



FIG. 18. Ciliated cells from 

 the windpipe. 



40. Safeguards of the body against 

 disease. In the first place, the 

 tough outer skin, as long as it is 

 unbroken, forms a most effective 

 barrier to the entrance of bacteria, 

 except at the mouth and nose 

 openings. Each of the nostrils is 

 guarded by hairs that collect a large 

 number of dirt particles. On the .mucous membrane lining 

 the nose and throat still other bacteria are caught, and the 

 cells which line the windpipe are furnished with cilia, which 

 lash upward (Fig. 18) and tend to expel the germs that may 

 have gone past the outer lines of defense that we have named. 

 If the bacteria enter the stomach and intestines in a living 

 condition, many of them are digested with the food. And 

 even though the invading microbes finally reach the interior 

 of the cells of our lungs, or muscles, or brain, we can still 

 rely upon the antitoxins which the cells of a healthy human 



