CHAPTER X 

 THE PHYSIOLOGY OF DIGESTION 



CHANGES UNDERGONE BY THE FOOD-STUFFS IN THE 

 ALIMENTARY CANAL 



THE use of the process of digestion is to alter the food- stuffs so as to fit 

 them for absorption into the blood, by means of which they may be carried 

 to all parts of the body. In most cases the food-stuffs cannot be utilised 

 in their original form by the living cells. When we nourish ourselves at 

 the expense of an animal or plant, we are taking in not only the current coin 

 of the organism which is being used for the supply of energy to its vital 

 processes, but also, and to a much larger extent, the framework forming 

 the machinery of the organism as well as its stores of carbohydrate or fat. 

 The food-stuffs as we ingest them are in the most inactive form possible. 

 Practically all are colloidal, neutral, and tasteless, and present no tendency 

 to unite with oxygen or, indeed, to undergo any change whatsoever, apart 

 from the interference of living organisms such as bacteria. In a starving 

 animal the stores of carbohydrate and fat and the protein structure of the 

 inactive living cells have to be converted into a soluble form transformed, 

 so to speak, into currency before they can be utilised by other living cells, 

 such as those of the heart, for the discharge of their normal functions and the 

 maintenance of the life of the animal. In the same way, when we take these 

 colloidal or insoluble substances into our alimentary canal, they have to be 

 rendered soluble or diffusible, in order to allow of their easy transference 

 across the wall of the gut into the blood and their transport to the tissue-cells. 

 The cells of the body cannot deal with all kinds of carbohydrate. Most 

 animal cells will starve when presented with starch, dextrin, or any of the 

 disaccharides, such as maltose, lactose, or cane sugar. It is necessary there- 

 fore that all the carbohydrates shall be reduced in the alimentary canal or in 

 its walls to the form of monosaccharides. As regards proteins, the processes 

 of digestion have a different significance according as we are dealing with their 

 value as givers of energy or their value as builders up of the living protoplasm. 

 If the proteins of the food are to be oxidised and utilised as a source of 

 energy, they must be rendered soluble so as to enable them to be absorbed 

 and carried to those parts of the body where they may undergo deamination 

 and complete oxidation. If they are to be built up as integral parts of 

 living cells, to take the place of molecules which have been destroyed in the 



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