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 
