CHAPTER III 



DIGESTION AND RESORPTION 



112. The first step in nutrition. The facts considered in 

 Part I have served incidentally to show some particulars of 

 those differences between the feed of herbivora and the animal 

 body which it serves to nourish which are, in a general way, 

 familiar to every one. The former contains many ingredients 

 not found in the latter, and it is plain that, for example, sub- 

 stances like starch and cellulose must undergo considerable 

 modification before they can be used in the animal organism. 

 One need not be a chemist, however, to reach this conclusion. 

 A simple comparison of the feeds given farm animals with the 

 products which they manufacture out of them convinces one 

 that profound changes are necessary to convert hay and grain 

 into meat or milk. The first step in this process is the diges- 

 tion of the feed. In all but the lowest animals, special tissues 

 are set apart for this work, together constituting the organs 

 of digestion, or the alimentary canal with its appendages. 



i. THE ORGANS OF DIGESTION 



113. General plan. The process of digestion is seen in its 

 simplest form in unicellular animals like the ameba. When 

 the ameba comes in contact with a particle of feed, a depres- 

 sion forms in its outer surface which finally closes around the 

 particle, forming a cavity which serves as a temporary digestive 

 organ. Undigested residues are rejected by the reverse pro- 

 cess. In animals slightly higher in the scale, this temporary 

 cavity becomes a permanent one, the same opening serving for 

 the entrance of feed and the exit of waste. The next step in 

 the evolution is the provision of a separate exit for the waste 

 matter, thus giving the typical form of digestive apparatus, 

 of which that of the higher animals is a development, consisting 



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