=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. =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. =The Master Tissues.=--The =muscular tissue= consists chiefly of rows of cells placed end to end (Fig. 10). These cells have the remarkable property of becoming broader and shorter when stimulated by impulses from nerve cells. [Illustration: FIG. 12.--CONNECTIVE TISSUE CELLS, removed from among the fibers of Fig. 13. _n_, _c_, nucleus; _p_, branches.] The =nerve tissue= consists of cells with long, spiderlike branches (Fig. 11). Some nerve cells have branches several feet long, so long that they go from the backbone to the foot. The branches are called _nerve fibers_ (Fig. 142). Nerve fibers which carry impulses _to_ the nerve cells are called _sensory fibers_. The nerve fibers which carry impulses _from_ the nerve cells are called _motor fibers_. The organs are set to work by impulses through the motor fibers. Besides these two master tissues there are five =supporting tissues=. [Illustration: FIG. 13.--CONNECTIVE TISSUE FIBERS. _a_, _b_, bundles of white fibers; _c_, a yellow fiber.] =Connective tissue=, like all other tissues, _contains cells_ (see Fig. 12), but it consists chiefly of fine fibers. These fibers are of two kinds,--very fine _white fibers which are inelastic_, and larger _yellow fibers which are very elastic_ (see Fig. 13). Connective tissue is found in every organ, binding together the other tissues and cells. It is interwoven among the muscle cells, and the tendons at the ends of the muscles are composed almost wholly of it. If every other tissue were removed, the connective tissue would still give a perfect model of all the organs. How abundant this tissue is in the skin may be known from the fact that leather consists entirely of it.