INTR OD UCTION 7 



surrounding the cell, called the cell wall. Workers with 

 the microscope found long ago that animals and plants are 

 constructed of little chambers which they called cells. It 

 was found later that the soft contents in the little chambers 

 is of more importance than the walls which the protoplasm 

 builds around itself. A living cell is not like a cell in a 

 honeycomb or a prison. In biology we define a cell as a 

 bit of protoplasm containing a nucleus. No smaller part of 

 living matter can live alone. The protoplasm of the nu- 

 cleus is called nucleoplasm ; the rest of the protoplasm is 

 called cytoplasm. 



A fiber is threadlike, and is either a slender cell (Fig. 8), 

 a slender row of cells (Fig. 10), or a branch of a cell. A 



Fig. 8. — A CELL (from involuntary muscle), so slender that it is called a fiber. 



tissue is defined as a network of fibers or a mass of similar 



cells serving the same purpose, or doing the same work. A 



membrane is a thin sheetlike tissue. 



The Nature of the Human Body. — The human body is a 



community of cells, and may be compared to a community 



of people. It is a crowded community, for all the citizens 



live side by side as they work. They are so small that it 



takes several hundred of them to make a line an inch long. 



We should never have suspected the existence of cells had 



it not been for the microscope ; but now we know that 



they eat and breathe and work and divide into young cells 



which take the place of the old ones. 



A child that is born in a community of people may become a railroad 

 man and carry food and other freight from place to place ; so, in the 

 great community of cells (see Fig. 9) making up the human body, the 

 red blood cells, like the railroad man, are employed in carrying material 

 from place to place. But the community is old-fashioned, for the 



