INFECTION, IMMUNITY, AND RECOVERY. 193 
in proportion as they are incapable of doing this they 
are assisted by the substances contained in the leuco- 
cytes. If the tissues are wholly adapted for the 
growth of the bacteria, neither they nor the leuco- 
cytes, nor both combined, can furnish sufficient pro- 
tective substances to prevent the bacterial increase. 
The entrance of bacteria into the leucocytes, which is 
not infrequent, may mean their destruction; but, on 
the other hand, the bacteria may increase in the white 
blood-cells and destroy them, and they may be killed 
without entering the cells. The simple absorption by 
the cells is uot necessarily a destructive process. No 
explanation can as yet be given of natural immunity 
to bacterial poisons, except that it may be connected 
with some general property of the tissue. There is far 
less variation among different species in their resistance 
to the bacterial poisons than in their suitability for the 
growth of the living bacteria which produce them 
Possibly certain organs, such as those which are rich 
in nuclein—for example, the lymph-glands, the liver, 
etc.—-may have some destructive power with regard to 
poisons. The nature of the cell substance is known 
to have much to do with its relations to certain poisons. 
Thus the tetanus poison acts chiefly on the nerve cells 
and leaves the others almost or altogether unaffected. 
By what means are virulent bacteria enabled to in- 
crease in the body, notwithstanding its protective powers, 
when non-virulent organisms of the same species are 
incapable of so doing? This is but little understood, 
but experiment shows in the first place that both viru- 
lent and non-virulent forms are equally resistant to 
general destructive agencies; and, second, that the bac- 
teria are capable of producing substances (lysines) which 
