114 PHYSIOLOGICAL ACTIVITIES 



enzymes are spoken of as extra-cellular or " exo-enzymes," 

 and in the latter as intra-cellular or " endo-enzymes." The 

 endo-enzymes and doubtless also the exo-enzymes may after 

 the death of the cell digest the contents to a greater or less 

 extent and thus furnish substances that are not otherwise 

 obtainable. This process of "self-digestion" is known tech- 

 nically as "autolysis." 



A distinction was formerly made between "organized" 

 and "unorganized ferments." The former term was applied 

 to the minute living organisms, bacteria, yeasts, molds, etc., 

 which bring about characteristic fermentative changes, 

 while the latter term was restricted to enzymes as just 

 described. Since investigation has shown that the changes 

 ascribed to the "organized ferments" are really due to their 

 enzjTnes, and that enzymes are probably formed by all liv- 

 ing cells, the distinction is scarcely necessary at present. 



PRODUCTION OF TOXINS. 



The injurious effects of pathogenic bacteria are due in 

 large part to the action of these substances, which in many 

 respects bear a close relationship to enzymes. The chemical 

 composition is unknown since no toxin has been prepared 

 "pure" as yet. It was formerly thought that they were 

 protein in character, but very pure toxins have been pre- 

 pared which failed to show the characteristic protein reac- 

 tions. It is well established that they are complex sub- 

 stances, of rather large molecule and are precipitated by 

 many of the reagents which precipitate proteins. Toxins 

 will be further discussed in Chapter XXVII. It will be 

 sufficient at this point to enumerate their chief peculiarities 

 in order to show their marked resemblance to enzymes. 



1. Toxins are dead organic chemical substances. 



2. They are always prbduced by living cells. 



3. They are active poisons in very small quaniifies} 



4. Their action is specific in that each toxin acts on a 

 particular kind of cell. The fact that a so-called toxin acts 



' Tetanus toxin is about 120 times as poisonous as strychnin, both of 

 which act on the same kind of nerve cells. 



