CHAPTER 8 

 THE DIGESTIVE SYSTEM 



Life depends upon an unceasing intake and outgo of matter. Each 

 living thing takes in food or the raw materials for food, assimilates this 

 into its own peculiar sorts of protoplasm, and after forming these chemical 

 substances, promptly burns them up into simpler chemical substances, 

 which finally leave the body as the wastes and ashes of life. Upon this 

 fundamental chemical process of metabolism, all other vital functions 

 depend. The foundations of life are chemical. 



The products of plant metabolism, on their way back to the inorganic 

 world, become, directly or indirectly, the food of animals. Thus all 

 animals are parasites on the green plants. But their feeding habits are 

 varied. Some marine organisms live on the mud as well as in it; earth- 

 worms pass through their digestive tract enormous quantities of soil for 

 the sake of the organic matter which they extract from it. But leeches 

 live chiefly on blood. Oysters sweep bacteria into their mouths by ciliary 

 action. Barnacles kick food into their mouths by means of their six 

 pairs of legs. Some insect larvae feed on cellulose, some on fur and wool. 

 Some whales eat minute swimming crustaceans, which they strain out by 

 means of the whalebone. Others live chiefly on gigantic cuttle-fish. 

 Some mammals are herbivorous; some are carnivorous; others, like man, 

 are omnivorous. Man alone cooks his food. 



Digestion. The first chemical change which ingested foods undergo 

 is a process by which insoluble substances are made soluble, so that they 

 may be absorbed through the lining membranes of the small intestine. 

 The agents in this chemical process are certain remarkable enzymes which, 

 like other and inorganic catalyzers, are able to bring about chemical 

 changes without appreciable effect on themselves. During digestion, 

 these enzymes split up the huge molecules of colloids into simpler mole- 

 cules, small enough to pass through animal membranes. Their composi- 

 tion is unknown; but they are thought to be rather simple colloids derived 

 from proteins. The specificity of their action is remarkable, each enzyme 

 affecting only one food substance. All are secreted by glands connected 

 with the alimentary canal. 



EVOLUTION OF THE DIGESTIVE SYSTEM 



The Protozoa have no digestive system. The single cefl merely 

 engulfs the food particle, surrounds it with protoplasm, digests and assim- 



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