BOOK VI 



THE PROCESSES OF DIGESTION 



CHAPTER XLIV 



THE MECHANISM OF THE SECRETION AND ACTIVATION OF 

 THE DIGESTIVE FLUIDS , 



FOR the proper digestion of the food, digestive juices are necessary. 

 These are provided either by large compound glands which lie wholly 

 outside the alimentary canal, and are connected to it by ducts, such as 

 the salivary glands, liver, and pancreas; or by glands which occur 

 in the lining membrane of the alimentary tract itself. These are 

 comparatively simple gland tubes, and line the whole of the stomach, 

 small and large intestine. The lining cells of the alimentary canal 

 also contribute to the secretion of mucus, which acts as a lubricant, 

 protects the mucous membrane from too high a concentration of 

 ingested material, and furthers the passage of contents down the gut. 



To understand rightly the processes concerned in the digestion of 

 food, it is necessary that we study (1) the mechanisms by which 

 the digestive fluids are provided; (2) the means by which the 

 enzymes they contain are activated or otherwise rendered efficient 

 digestive agents. 



The proper preparation of foodstuffs for digestion, and their 

 adequate digestion, are matters of first importance to the general 

 well-being of the body. Discomfort and local pain occur when these 

 functions are temporarily deranged ; malnutrition, anaemia, depression 

 of spirits, and general ill-health, follow chronic indigestion. 



Digestion of the food is necessary, in the first place, in order to 

 convert the complex, colloidal, non-diffusible, and insoluble protein, 

 starch, and the fat, into simpler soluble, diffusible, and non-colloidal 

 compounds, which are absorbed by the cells lining the alimentary 

 tract. Secondly, it is necessary because some of the component parts 

 of the food material introduced into the body are of little or no value 

 to the body; others are of intermediate value; others, again, are so 

 precious that without them the body cannot live. In order, there- 

 fore, that these components may be sorted according to their true value, 

 it is necessary that they be separated from each other by the hydro - 

 lyzing action of the digestive enzymes. 



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