FOOD AND DIGESTIOK. 331 



heat is to produce a scuin composed of albumen and a little caseinogen 

 (the greater part of the caseinogen being uncoagulated) with some fat. 

 Upon (d ) vegetables, the cooking produces the necessary effect of render- 

 ing them softer, so that they can be more readily broken up in the mouth ; 

 it also causes the starch grains to swell up and burst, and so aids the 

 digestive fluids in penetrating into their substance. The albuminous 

 matters are coagulated, and the gummy, saccharine and saline matters 

 are removed. The conversion of flour into dough is effected by mixing it 

 with water, and adding a little salt and a certain amount of yeast. Yeast 

 consists of the cells of an organized ferment (Torula cerevisice), and it is 

 by the growth of this plant, changing by ferment action the sugar pro- 

 duced from the starch of the flour, that a quantity of carbonic acid gas 

 and alcohol is formed. By means of the former the dough rises. An- 

 other method of making dough consists in mixing the flour with water 

 containing a large quantity of carbonic acid gas in solution. 



By the action of heat during baking (d ) the dough continues to ex- 

 pand, and the gluten being coagulated, the bread sets as a permanently 

 vesiculated mass. 



Digestion. 



The Enzymes, or unorganized ferments, are the essential factors 

 in digestion, and 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. Their chemical nature 

 is as yet undetermined because of the inability of getting absolutely pure 

 specimens, but it is generally admitted that they contain nitrogen, and 

 they are usually classed as proteids. 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. Some of them pass into the urine, but 

 most are excreted with the faeces. 



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 sus- 

 pended at a definite point of low temperature, but the enzyme is not de- 

 stroyed by cold ; the action is also suspended at higher temperatures, and 

 at a still higher point the enzyme is destroyed. Some enzymes act only 

 in an alkaline medium, being destroyed in an acid medium, and vice versa ; 

 others act in either alkaline, neutral or acid media. Enzymes are hin- 

 dered in their action by the accumulation of the products of their activity. 

 Most of them cease acting altogether when these products reach a certain 

 concentration, but will begin acting again on the removal of these prod- 

 ucts or if the mixture be simply diluted. 



