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Fig. 10.—Three Muscle Fibers from the heart (showing the nuclei of six cells).
The Tissues.—As the organs have different functions, they must have different structures that they may be adapted to their work. Just as a house must have brick for the chimney, shingles for the roof, and nails to hold the timbers and other parts together, so the body has various tissues to serve different purposes. The bones must not be constructed like the muscles, and the muscles cannot be like the skin. The chief work of the cells is to construct the tissues and repair them. During life changes are constantly going on. Careful little workmen are keeping watch over every part of the body; thrifty little builders are busy in repairing and restoring. No sooner is one particle removed than another takes its place. In one direction the cells, acting as undertakers, are hurrying away matter which is dead; in the other direction the unseen builders are filling the vacant places with matter that is living.
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Fig. 11.—Nerve Cells, showing their branches interlacing.
The Seven Tissues.—There are seven kinds of tissues. Two of them, the muscular and nervous tissues, are called the master tissues, since they control and expend the energies of the body. The other five tissues are called the supporting tissues, since they supply the energy to the master tissues, support them in place, nourish and protect them.
