122 BACTERIOLOGY. 



protective bodies which warded off disease, and that 

 they did this by attacking the bacteria, was founded 

 on the observation that certain of the white cells pos- 

 sessed the power of taking up into themselves patho- 

 genic bacteria, and that they were there destroyed. It 

 was later observed that these cells had the property of 

 taking from the blood many lifeless foreign elements, 

 thereby keeping the blood-channels free of foreign par- 

 ticles. The question thereby arose as to whether these 

 cells engulfed and then killed the bacteria, or whether 

 perhaps other influences killed or injured them before 

 the cells took them up. The theory then became some- 

 what modified, more knowledge was obtained, and it is 

 now believed that the bacterial substances attract the 

 cells, and that when these cells are brought together the 

 general, and perhaps the specific, bactericidal property 

 of the blood in their neighborhood is thereby increased. 

 The death of the bacteria liberates this positive chemo- 

 taxic substance, and the disintegration of the white 

 blood-cells gives rise to the bactericidal bodies. Thus 

 we find that phagocytosis is most marked when the dis- 

 ease is on the decline or the infection mild, but that in 

 rapidly increasing progressive infection it is absent. 

 This would seem to indicate that the course of the in- 

 fection is often already determined before the leucocytes 

 become massed at the point of its entrance. The first 

 determining influence is given by the condition of the 

 tissues and the bactericidal substances contained in 

 them, and then, later, in cases where the infection is 

 checked, comes the additional bactericidal substance 

 given off by the attracted leucocytes. In so far as the 

 tissues themselves are unsuitable for the development 

 of bacteria they are sufficient to ward off infection, but 



