A STUDY OF LIVING SUBSTANCE 29 



Production of Energy. Between a working locomotive and 

 our bodies, as already noted, there are many points of re- 

 semblance. The building materials of an engine are brass, 

 wood, glass, and iron ; these may be regarded as the tissues 

 of the machine. A combination of several of these materials 

 form the boiler, the wheels, the whistle, and the headlight ; 

 and since each of these parts of the engine has some special 

 work to do, we may call them organs. Again, like our bodies, 

 the engine must be continually supplied with water and fuel 

 in order to do its work. As coal is burned in the locomotive 

 to give heat and power, so food is oxidized in animals ; and 

 in both kinds of machines waste materials are formed and 

 thrown off. 



Growth. But the comparison between our bodies and a 

 locomotive must not be carried too far. In the first place, 

 no one ever knew of an engine to begin its existence as a 

 small machine and then to increase in size little by little 

 until its weight had multiplied twenty times. Yet this 

 is true of the human body. An average child at birth weighs 

 about seven pounds ; the weight of a grown man commonly 

 exceeds one hundred and forty pounds. None of the coal 

 and water put into the engine is changed into the brass, iron, 

 or other " tissue " of the machine. But in the human body 

 a great part of the food we eat becomes muscle, bone, and 

 brain, for these animal tissues in some unknown way can 

 make over lifeless food materials into living substance. One 

 of the most striking properties of protoplasm is this power 

 to make more protoplasm, or in other words, to grow. To 

 this process is given the name as-sim-i-la'tion (Latin ad = 

 to + similis = like) ; for when muscle tissue, for example, 

 takes from the blood its supply of food, the latter is made 

 by the muscle into protoplasm like to itself. 



Repair By continual use parts of the locomotive become 

 worn or broken, and the engine must go to the machine shop 

 for repairs. In our bodies, too, the tissues are being con- 

 stantly worn away. Every time we use our muscles some of 



