=A living cell= _always contains a still smaller body called a_ =nucleus= (Fig. 7). There is sometimes a small dot in the nucleus, called the _nucleolus_. _The main body of the cell consists of the living substance called_ =protoplasm=, _containing nitrogen_. Usually, but not always, there is a wall 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. [Illustration: 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.