THE PHENOMENA OF INFECTION 453 



activity distinguish living from non-living matter. We 

 have taken the leukocyte as an illustration, but the bac- 

 terium is also supplied with its ferments, some of which are 

 intra- while others are extracellular. We do not know that 

 all cells elaborate both kinds of ferments, but all have at 

 least one kind. 



Before proceeding further it may be well to call special 

 attention to some of the properties of these ferments. The 

 extracellular ferments are diffusible. They not only pass 

 out of the cells in which they are prepared; but they diffuse 

 more or less widely through the medium which surrounds 

 the cell. This suggests that in molecular structure they 

 are relatively simple. At least some of them may pass 

 through membranes and collodion sacs, as is shown by the 

 fact that bacteria and other proteins enclosed in such 

 receptacles and left in a body cavity are destroyed. 

 The extracellular ferments are, in part at least, filterable, 

 passing with more or less readiness through porcelain. 

 In their activities they are easily affected by modification 

 in the medium through which they diffuse. The alexin 

 of the blood serum is highly sensitive to the salt content 

 of the serum, and by variations in this the activity of the 

 ferment may be hastened, lowered, or wholly arrested. 

 The same is true of bacterial ferments. In one species of 

 animal a given bacterium multiplies with great rapidity; 

 in another it grows slowly, while in a third it cannot grow 

 at all. There are like variations in individuals of the same 

 species. The extracellular ferments, at least some of them, 

 are susceptible to slight changes in temperature. It is 

 believed that every ferment has its optimum temperature, 

 but the range in which continued activity is possible is 

 narrow with some and relatively wide with others. 



The intracellular ferments are non-diffusible, or at least 

 less diffusible than the extracellular. They remain in the 

 cells in which they are elaborated. They cannot be extracted 

 from the cell by indifferent solvents. As a rule, they can 

 be obtained from the cell only after partial or complete 

 destruction of the cell. Some, probably most, are best 



