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 citizens build canals instead of railroads for their commerce (see Fig. 84). Just as a child may grow up to be a farmer and aid in the conversion 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 enemies. The white 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 enemies by devouring or digesting them. At other times they help to repair a break in the skin. If a splinter 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 referred to as a kind of Jack-at-all-trades. In the human community there are certain persons who reach the positions of teachers, lawmakers, 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. 11) 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.

of the body. (Jegi.) Tiny citizens of the bodily community.

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 this system each cell can do its own work better than a cell of any other kind can do it. Among wild tribes