114 BACTERIA 



always due to a living agent, as proved by Pasteur, the 

 process is conveniently divided into two kinds. 1 (i) When 

 the action is direct, and the chemical changes involved in 

 the process occur only in the presence of the cell, the latter 

 is spoken of as an organised ferment ; (2) when the action is 

 indirect, and the changes are the result of the presence of a 

 soluble material secreted by the cell, acting apart from the 

 cell, this soluble substance is termed an unorganised soluble 

 ferment , or enzyme. The organised ferments are bacteria or 

 vegetable cells allied to the bacteria; the unorganised fer- 

 ments, or enzymes, are ferments found in the secretions of 

 specialised cells of the higher plants and animals. With the 

 former this book deals in an elementary fashion ; with the 

 latter we have little concern. It will be sufficient to illus- 

 trate the enzymes by a few of the more familiar examples. 

 They form, for example, the digestive agents in human as- 

 similation. This function is performed, in some cases, by the 

 enzyme combining with the substance on which it is acting 

 and then by decomposition yielding the new " digested " 

 substance and regenerating the enzyme ; in other cases, the 

 enzyme, by its molecular movement, sets up molecular 

 movement in the substance it is digesting, and thus changes 

 its condition. These digestive enzymes are as follows : in 

 the saliva, ptyalin, which changes starch into sugar; in the 

 gastric juice of the stomach, pepsin, which digests the pro- 

 teids of the food and changes them into absorptive pep- 

 tones; the pancreatic ferments, amylopsin, trypsin, and 

 steapsin, capable of attacking all three classes of food stuffs; 

 and the intestinal ferments, which have not yet been sepa- 

 rated in purer condition. In addition to these, there are 

 ferments in bitter almonds, mustard, etc. Concerning 

 these unorganised ferments we have nothing further to say. 

 Perhaps the commonest of them all is diastase, which occurs 

 in malt, and to which some reference will be made later. 



1 E. A. Schafer, F.R.S., Text-book on Physiology, vol. i., p. 312. 



