XIX 

 PROTOPLASM, LIFE AND DEATH 



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 move- 

 ment, change of shape, and internal currents of granules, 

 as well as by the special chemical substances it pro- 

 duces 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 pro- 

 ducts 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 con- 

 verted 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 



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