

CHAPTER X. 

 THE DIGESTIVE SYSTEM. 



SECTION I. 

 THE NATURE OF DIGESTION. 



THE process of digestion consists essentially in the splitting up of the 

 molecules of the food-stuffs into a large number of much smaller 

 molecules which, partly because of their smaller mass, are easily 

 absorbed through the mucous membrane of the digestive tract. Thus 

 the digestion of fat results in the splitting up of the molecule of neutral 

 fat into one molecule of glycerol and three molecules of fatty acid, a 

 single protein molecule is broken up into a very large number, probably 

 a hundred or more, of molecules of amino-acids, while one molecule of 

 starch is subdivided into about two hundred molecules of dextrose. In 

 all cases the process is one of hydrolysis, and in the case of protein and 

 starch it takes place in a series of stages. Similar changes can be 

 brought about by chemical means, such as boiling with mineral acids, 

 but the digestive juices achieve their results more rapidly and 

 effectively, at the temperature of the body, by means of certain active 

 agents known as enzymes or ferments. 



ENZYMES. 



No enzyme has yet been isolated and analysed, and therefore 

 nothing definite is known as to the composition and constitution of 

 these bodies except that they are not proteins. They exist, however, 

 in great variety in animal and vegetable cells, and they possess very 

 definite properties. (1) They are colloidal substances, and do not 

 diffuse through animal membranes. (2) They only act in solutions, 

 having no effect in the dry state. (3) As a rule each enzyme is 

 specific ; it will produce its own definite effect and no other. Thus the 

 ptyalin of saliva converts starch into maltose, but has no action on 

 protein. (4) Enzyme action is markedly affected by temperature. It 

 is most' effective, in the case of the enzymes of the animal body, at 



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