METABOLISM 



35 



6/ycer/n J<^'^^s) 



/no adds 

 (Protein) 



to learn these words in this connection as scientific terms, each with a 

 very precise meaning, and to keep this meaning separate from that which 

 he may have hitherto attached to the word. 



48. Ingestion. — The first step in metaboUsm is the taking in of food — 

 ingestion. This may occur in certain one-celled animals through any 

 point on the surface of the body. While the same may be true to a cer- 

 tain degree in the case of higher animals, most of them, and some of the 

 one-celled ones, take food through a particular opening on the surface of 

 the body, called the mouth. Under 

 the head of ingestion also occurs all 

 mechanical processes such as chewing 

 and swallowing which precede any 

 chemical change. 



49. Digestion. — As soon as the 

 food is in position to be acted upon (clrbSi ydrak), 

 by digestive fluids, these are secreted ' 

 into the cavity which contains it. By 

 their action a series of chemical and 

 physical changes is initiated, which 

 results in reducing the solid food to 

 liquid form and changing part of it 

 chemically so as to render it capable of 

 being absorbed. This process is called 

 digestion. In the higher animals diges- 

 tion may begin in the mouth and be 

 continued in the stomach and intes- 

 tine. Only organic foods need to be 

 digested, the other foods being capable of absorption without undergoing 

 this process. 



50. Absorption. — The passage of the digested food from the food 

 vacuole into the protoplasm of one-celled animals is termed absorption. 

 In higher animals the same process takes place by the food entering the 

 cells forming the lining of the alimentary canal and being then passed 

 into the blood or lymph contained in blood vessels or lymphatics which 

 lie behind these cells (Fig. 11). In vertebrates absorption occurs mostly 

 in the small intestine. The digested food is not further changed during 

 this process, though it may suffer a change as soon as the process is 

 complete. In the process of digestion fats, for example, are broken down 

 into fatty acids and glycerin but are changed back to fat in the cells into 

 which they are absorbed. 



51. Circulation. — Whether the animal is one- or many-celled, the 

 food cannot be all utilized at the point of absorption but must be cir- 

 culated throughout the living body for use in various parts. This 

 circulation may take place within the cell, by osmosis from cell to cell, 



Fig. 11. — Diagram of a villus, one of 

 the finger-like projections in the small 

 intestine of a mammal, to show how the 

 absorption of organic foods takes place. 

 The lymph vessels are in solid black, the 

 blood vessels stippled. 



