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. Jn 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 transfer- 

 ence 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 carbo- 

 hydrate. 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 monosac- 

 charides. 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 ab- 

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



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