8o4 THE POPULAR SCIENCE MONTHLY. 



useless concomitants. The latter bodies are rejected at once ; but the 

 food-stuffs are taken up by his veins, incorporated with the blood 

 (which consists of food in different degrees of combustion), and used 

 for building up the various portions of his body. Supposing the ani- 

 mal were a mere growing object like a crystal, with no work to perform 

 and no consequent waste of material, the process would stop here, and 

 the creature would wax bigger and bigger from day to day, without any 

 alteration in place or redistribution of assimilated matter. But the 

 animal is essentially a locomotive machine, and the purpose for which 

 he has taken in his food is simply that he may use it up in producing 

 motion. For a while he stores it away in his muscles, or lays it by for 

 future use as fat ; but its ultimate destination in every instance is just 

 as truly to be consumed for fuel as is the case with the coal in the 

 steam-engine. 



The food, however, only gives us one half of the necessary materials 

 for the liberation of dormant energy. Oxygen is needed to give us the 

 other half. This oxygen we take in whenever we breathe. Animals 

 like fishes or sea-snails obtain the necessary supply from the water by 

 means of gills ; for large quantities of oxj-gen are held in solution by 

 water, and the needs of such comparatively sluggish creatures are not 

 very great. With them a little energy goes a long way. Air-breath- 

 ing animals like ourselves, on the other hand, need relatively large 

 quantities of the energy-yielding gas in order to keep up the constant 

 movements and high temperature of their bodies. Such creatures, ac- 

 cordingly, take in the oxygen by great inhalations, and absorb it in 

 their lungs, where it passes through the thin membrane of the capil- 

 laries, or very tiny blood-vessels, and so mixes freely with the blood 

 itself. Thus we have food, supplied to the blood by the stomach, the 

 exact analogue of the coal in the engine ; and oxygen, supplied to the 

 blood by the lungs, the exact analogue of the draught in the engine. 

 Whenever these two substances — the hydrocarbonaceous foods and 

 the free oxygen — reunite, they will necessarily give out heat and set 

 up active movements. 



The exact place and mode of their recombination we can not yet be 

 said to fully understand. But even if we did, the details would be suf- 

 ficiently dry and uninteresting to general readers ; and we know quite 

 enough to put the subject in a simple and comprehensible form before 

 those who are willing to accept the broad facts without small criticism. 



We may say, then, that the energies of the body are used up in two 

 principal ways — automatically and voluntarily. The automatic activi- 

 ties are produced by the steady and constant oxidation of some portion 

 of the food-stuffs in the blood and tissues. As this oxidation takes 

 place, it sets up certain regular movements, which compose what is 

 (very incorrectly) known as the vegetative life in animals. There are 

 an immense number of these movements always going on within our 

 bodies, quite apart from our knowledge or will. Such are the beating 



