CHAPTER X 

 THE PHYSIOLOGY OF DIGESTION 



CHANGES UNDERGONE BY THE FOODSTUFFS IN THE 

 ALIMENTARY CANAL 



THE use of the process of digestion is to alter the foodstuffs 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 foodstuffs 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 foodstuffs 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 trans- 

 ference 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 

 therefore 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 



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