8 



HUMAN BIOLOGY 



citizens build canals instead of railroads for their commerce (see Fig. 

 84). Just as a child may grow up to be ^farmer and aid in the con- 

 version of crude soil into things suitable for the use of man, so the 

 digestive cells take the food we eat and change it into material with 

 which the cells can build tissue. Some of the citizens of a community 

 must, at times, take the part of soldiers and policemen, and protect the 



community against 

 the attacks of ene- 

 mies. The ivhite blood 

 cells, already referred 

 to, may be called the 

 soldiers ; for they go 

 to any part attacked 

 by injurious germs, a 

 particle of poison, or 

 other enemy, and try 

 to destroy the ene- 



sSSi^SiS^^^ g/fc mies by devouring or 



digesting them. At 

 other times they help 

 to repair a break in 

 the skin. If a splin- 

 ter gets into the skin, the white blood cells form a white pus around 

 the splinter and remove it. In fact, the white blood cell has been re- 

 ferred to as a kind of Jack-at-all-trades. In the human community 

 there are certain persons who reach the positions of teachers, law- 

 makers, and governors ; they instruct and direct the other members of 

 the community. Just so, in the community of cells, there are certain 

 cells called nerve cells (see Fig. n) that have the duty of governing 

 and directing the other cells. The nerve cells are most abundant in 

 the brain. Large cities must have scavengers. Likewise in the human 

 body, a community composed of millions of cells, there are certain cells 

 in the skin and the kidneys which have this duty. They are continually 

 removing impurities from the body. 1 



Division of Labor. There is a great advantage in each 

 cell of the human body having its special work, instead of 

 having to do everything for itself, as each ameba cell must 

 do. Under tJiis system each cell can do its own work better 

 than a cell of any other kind can do it. Among wild tribes 



1 From Coleman's " Hygienic Physiology," The Macmillan Co., N.Y. 



FIG. 9. VARIOUS CELLS of the body. (Jegi.) 

 Tiny citizens of the bodily community. 



