138 TEXT-BOOK OF PHYSIOLOGY 



with certain waste products, pass into the large intestine to be finally ex- 

 truded from the body. 



FERMENTS; ENZYMES 



In a preceding chapter it was stated that under favorable conditions 

 the carbohydrates, fats, and proteins undergo reduction to simpler com- 

 pounds as a result of the action of agents such as the yeast plant and various 

 forms of bacteria. To this process of reduction the term fermentation, and 

 to the agent which causes the fermentation the term ferment, or enzyme has 

 been given. As these compounds undergo reduction to simpler substances 

 somewhat different in character in the alimentary canal during the period of 

 digestion as a result of the action of ferments, it will be conducive to clearness 

 of ideas regarding the nature of the digestive process if the nature and prop- 

 erties of ferments in general are briefly considered at this time. 



A ferment or an enzyme may be defined as an agent that induces a change 

 of state, or a change in composition of an organic compound, for the most 

 part hydrolytic, without itself being utilized in the process or appearing in 

 the end-results of the process. 



Ferments have been divided for a long time into two classes, viz., organ- 

 ized and unorganized. Among the organized ferments may be mentioned 

 the yeast plant (Saccharomycetes) and various forms of bacteria; among 

 the unorganized ferments may be mentioned the diastase that transforms 

 the starch of barley, wheat, or other cereals into sugar, as well as ptyalin, 

 pepsin, steapsin or lipase, and other ferments contained in the digestive 

 fluids that transform or reduce the food principles to simpler compounds. 



It will be recalled that if the yeast plant is added to a sugar solution 

 containing in addition some protein and various inorganic salts such as 

 phosphates and the solution kept at a favorable temperature the yeast cells 

 soon begin to grow and multiply. Coincidently the sugar is reduced for 

 the most part to carbon dioxid and alcohol. The carbon dioxid bubbling 

 through the solution as steam bubbles through water that is boiling, gave rise 

 to the expression fermentation (from fervere, to boil), and as this was 

 attributed to the life activities of the yeast plant it was called a ferment. 



Again, if dead protein matter is exposed to air and moisture at a suitable 

 temperature it will be invaded by various species of bacteria, which in a short 

 time will begin to grow and multiply. Coincidently the protein molecules are 

 reduced to simpler compounds, such as hydrogen sulphid, ammonia, 

 carbon dioxid and a number of other compounds, the nature of which will 

 vary with the character of the protein. As this reduction is accompanied by 

 the bubbling of gases through the surrounding liquid, it too has received the 

 name of fermentation, and as the reduction is attributed to the life activities 

 of the bacteria they too have been called ferments. In both instances the 

 ferment is a unicellular plant possessing a distinct organization. For this 

 reason they have been termed organized ferments. 



When grains of barley or other cereals containing starch are exposed to 

 moisture and a suitable temperature, the starch is gradually changed to 

 sugar, a transformation attributed to the action of a ferment. When the 

 starches, fats, proteins, and compound sugars are introduced into the 

 alimentary canal they are also reduced to simpler compounds, a reduction 

 attributed to the action of a series of distinct and specific ferments. In 



