CHAPTER VIII 

 DIGESTION 



Introduction 



Food, in the form in which it is taken in by the mouth, 

 is incapable of being of service to the body : first, because 

 it is usually insoluble ; secondly, because even if soluble it 

 is not in such a form as to be absorbed by the gut. To 

 reduce the food into small molecules and to absorb these 

 into the blood-streajn are the functions of the digestive 

 system. 



In the process of digestion three physiological mechanisms 

 may be said to be involved : the secretion of digestive 

 juices, the action of enzymes upon the food, and the 

 movements of the alimentary canal. As regards enzyme 

 action we need only say here that this is invariably of the 

 nature of hydrolysis, and that in the changes thus pro- 

 duced in the food-molecules there is no loss of potential 

 energy. As regards the movements of the gut, these 

 comprise an orderly sequence of co-ordinated movements. 

 They serve to mix the food with the digestive juices, to 

 propel the digesting mass along the canal, to expose it to 

 the absorbing surface, and finally to evacuate such remnants 

 as are not absorbed. As regards the secretion of digestive 

 juices, some general remarks are necessary at this stage. 



The Nature of Secretion 



The formation of a secretion by a gland is associated 

 with certain histological changes. When the fresh gland 



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