THE PROCESS OF DIGESTION 327 



organized ferment (Torula cerevisia) ; this plant in its growth changes by 

 ferment action the sugar produced from the starch of the flour, and a quantity 

 of carbon dioxide and alcohol is formed; the gas together with the action of 

 heat during baking causes the dough to rise, and the gluten being coagulated, 

 the bread sets as a permanently vesiculated mass. 



THE PROCESS OF DIGESTION. 



The Enzymes. The digestive process involves both mechanical and 

 chemical changes. The former are secured by the crushing and grinding 

 in the mouth, together with the mixing and kneading that come from the 

 peristalses of the stomach and intestine. The chemical changes are the 

 most important factors of the digestive process. The various secretions that 

 are poured into the mouth, stomach, and intestines all contain substances 

 which react on the foods to render the latter more soluble. The special 

 agency in each secretion is the presence of representatives of the chemical 

 groups known as enzymes. These enzymes, or unorganized ferments, are 

 the essential factors in the secretions which produce the chemical changes 

 in the foods. Their predominant action is one of hydrolytic cleavage; that 

 is, the substance acted upon takes up water and then splits into two different 

 substances, usually of the same class. The chemical nature of the en- 

 zymes is as yet undetermined because of the difficulty of getting absolutely 

 pure specimens. Their mode of action is at present regarded in the nature 

 of catalysis. That is to say, the enzymes by their presence facilitate reactions 

 that would otherwise take place but very slowly. Practically all are secreted 

 in the glands as zymogens, which bear the same relation to enzymes as 

 fibrinogen does to fibrin; they are transformed to enzymes by the proper 

 stimulus, but never exist as such in the glands. 



Each enzyme has a special point of temperature at which it acts best, and 

 any change in the temperature retards its action; the action is suspended at 

 a definite point of low temperature, but the enzyme is not destroyed by cold. 

 The action is suspended at a somewhat higher temperature, and at a still 

 higher point the enzyme is destroyed. Some enzymes act only in an alka- 

 line medium, being destroyed in an acid medium, and vice versa. Others 

 act in either alkaline, or neutral, or acid media. Enzymes are hindered in 

 their action by the accumulation of the products of their activity. Most of 

 them cease acting altogether when these products reach a certain concen- 

 tration, but will begin acting again on the removal of these products or if 

 the mixture be simply diluted. 



The quantity of the enzyme determines the rapidity of the action, but not 

 the amount; a small quantity will digest as much as a large quantity, but will 

 take longer. The enzymes are not used up in the course of their activity, 

 as far as can be seen, and do not seem to undergo any change in their 

 composition. 



