CHAPTEE III 



MECHANICS AND CHEMISTRY OF DIGESTION IN THE MOUTH 

 AND STOMACH 



CONTENTS. 1. Historical. 2. Mastication, insalivation, formation of aliment- 

 ary bolus, and saccharification of starch. 3. Mechanism of deglutition. 4. Innerva- 

 tion. 5. Artificial digestion in vitro to determine action of gastric juice on different 

 food-stuff's. 6. Influence of spleen on gastric digestion. 7. Natural digestion 

 in the stomach. 8. Effects of total gastrotomy. 9. Active movements of 

 stomach in gastric digestion. 10. Mechanism of vomiting. 11. Peripheral and 

 central innervation of stomach. Bibliography. 



THE term " Digestion " usually denotes the complex of mechanical 

 and chemical changes effected in the food -stuffs by the muscular 

 tissue of the gastro-iutestinal canal and by the secretions of the 

 glands discussed in the last chapter. By these changes the food- 

 stuffs are reduced to the form necessary for their rapid absorption, 

 assimilation, and transference to the blood stream. 



By "food -stuffs" in the widest sense, we mean all those 

 substances which are normally contained in the blood plasma, 

 or can readily be converted into the same, and which do 

 not represent the end-products (or metabolites) of tissue 

 consumption, destined as such to be eliminated from the body. 

 A perfect diet' must therefore contain (a) protein; (5) fats; (c) 

 carbohydrates ; (d) water ; (e) various salts, the bases of which 

 are Na, K, Ca, Mg, Fe, and hydrochloric, sulphuric, and phosphoric 

 acid. The three groups of organic substances (proteins, fats, carbo- 

 hydrates) are oxidisable, i.e. they contain potential energy which 

 is greater or less in proportion to their capacity for oxygen ; the 

 mineral constituents (water and salts), on the contrary, are not 

 capable of being oxidised, and are therefore useless as sources of 

 energy, and merely fulfil the role of common solvent, or passive 

 material of construction. We shall discuss the physiological 

 classification of foods, according to the functions of each group of 

 substances (which is fundamental to the theory of nutrition'), 

 elsewhere, when we consider metabolism, i.e. the material 

 exchanges of the body as a whole. 



Of the oxidisable organic substances on which we subsist, sugar 



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