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 nucleus is called nucleoplasm; the rest of the protoplasm is called cytoplasm.
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Fig. 8.—A Cell (from involuntary muscle), so slender that it is called a fiber.
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 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
