The result of the study of living cell-substance, or protoplasm, is to show that every cell has an individual life, and often makes this manifest by its movement, change of shape, and internal currents of granules, as well as by the special chemical substances it produces and consumes. All depend for their activity upon the presence of free oxygen; all are killed by heat far less than that of boiling water; they continually imbibe water charged with the chemical substances which nourish them and cause them to grow in bulk and to divide into two; and they manufacture various chemical bodies in the protoplasm and emit heat, electrical discharges, and sometimes light. Some or other of them, in fact, do in their small microscopic way all that the complex, big animal or plant, of which they are constituents, is seen to do. The cells of the liver manufacture the bile, those of the salivary glands the saliva, and those of the intestinal wall a mucous fluid, and squeeze out or eject those products into the adjacent ducts (see Fig. 40 C). Other cells lay down (as cell-wall or coating) fibrous and hard substances which form the skeleton; others become converted into horn and are shed from the surface of the skin in man as “scurf”; others form the great contractile masses called muscles. One lot are told off to control the other cells by something resembling a system of electrical wires and batteries—these are the nerve-cells (Fig. 37 D), with their fine, thread-like branches, the nerve-fibres, which are long enough to permeate every part of the body and place it in connection with the nerve-cells in the great centres called brain, spinal cord, and ganglia.