72 PRINCIPLES OF PHYSIOLOGY 



In a similar way all enzymes are formed in 

 cells, as, for example, ptyalin in certain cells 

 of the salivary glands, and pepsin in certain 

 cells in the tubular glands in the mucous 

 membrane lining the stomach. 



32. The enzyme, however, is preceded in 

 the cell by an enzyme-forming substance, a 

 zymogen, as it is called, and while the cell is 

 secreting, matter appears in the form of small 

 granules. Thus ptyalin is preceded by pty- 

 alinogen, pepsin by pepsinogen, and so on. 

 It would appear that just when the secretion 

 is poured out the zymogen is changed into 

 the enzyme, and the enzyme at once begins 

 to act. Each enzyme has a limited field of 

 activity. Thus three are required to modify 

 the three varieties of di-saccharides, cane 

 sugar, milk sugar (lactose) and maltose, one 

 and only one for each sugar. The activity of 

 enzyme is greatest at about 40 C : at about 

 50 C. the ferment is destroyed Extreme 

 cold arrests the activity, but it does not appear 

 to injure it, as it will again act at, say, 40 C. 

 One remarkable feature of the action of an 

 enzyme is that only a small amount is neces- 



