I 



i 



PART VIII 

 METABOLISM 



SECTION XXVI 

 DIGESTION 



CHAPTER LXXXIII 



THE CHEMISTRY OF DIGESTION 



General Consideration. — The term of assimilation as originally 

 employed by the botanists, included all those processes which the 

 plants must undergo in order to synthetize the inorganic substances 

 into the organic compound starch. When employed in this general 

 way, it embraces all those chemical and mechanical processes which 

 lead to the reduction, absorption and assimilation of the different 

 foodstuffs by the cells. These stages are followed by cellular dissimi- 

 lation and excretion. At the present time, all these processes are 

 generally included under the term of metabohsm which is divided in 

 turn into a process of building up, or anabolism, and a process of tear- 

 ing down, or catabohsm, as follows : 



C Ingestion 



. , T J Digestion 



Anabolism < au 



Metabolism 



Catabolism 



j Absorption 

 I Assimilation 



Dissimilation 

 Excretion 



In the lower forms, the process of digestion is completed outside 

 the cells, enabling them to attain their nutritive material in a fluid 

 condition ready for assimilation. Beginning with the celenterates, 

 on the other hand, digestion is chiefly intracellular and hence, the 

 metabolism of the higher forms requires the presence of special organs, 

 the purpose of which is to reduce the nutritive material sufficiently 

 to render it dialyzable through animal membranes, and assimilable 

 by the cells. But since the food must be reduced mechanically as well 

 as chemically, two types of organs must really be present which sever- 

 ally accomplish these ends. In most instances, however, the mechan- 



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