NATURAL RESISTANCE TO DISEASE—FLEXNER. Tol 
they can exert their action. The toxin of the dysentery bacillus passes 
readily in the rabbit from the blood into the intestine, which it injures, 
but can not pass from the intestine into the blood. Tetanus toxin can 
be injected into the circulation of the hen, but doesnoharm. Injected 
into the brain it produces tetanus. Introduced into the blood it re- 
mains there for many weeks, hence the failure to act can not be due 
to destruction, but probably is due to inability to pass through the 
blood vessels in order to reach the cells of the central nervous system 
in a sufficient state of concentration. The physiological state of the 
animal also exerts an influence—certain hibernating species are sus- 
ceptible to tetanus poison in the summer, but not during the winter 
sleep. There exist, therefore, different mechanisms for excluding 
poisons from the sensitive and reacting cells, and among them are 
certain quantities of neutralizing, or antitoxic substances, normally 
contained in the blood. We know at least one such definite antitoxin, 
namely, the diphtheria antitoxin, which exists in minimal quantities 
in the blood of man and the horse. 
The absence of numerical relation between the mechanism which 
destroys bacteria and neutralizes poisons sometimes works sad havoc 
for the body. The two capacities may differ naturally or are en- 
hanced in different degrees by artificial means. The matter is one 
of great importance, because almost without exception all bacterial 
diseases are examples of poisoning. The mechanical obstructions 
produced by the bacterial bodies are relatively unimportant. The 
body is more readily defended from the invasion of bacteria, with 
very few exceptions, than from the effects of their poisons. The 
capacity to dispose of typhoid and cholera bacilli is more easily pro- 
duced than the power to neutralize or otherwise render innocuous 
the poisons liberated by the dissolved bacilli. It is precisely because 
we have not yet learned how to overcome this class of bacterial poi- 
sons within the body that we have not mastered the bacterial diseases 
as a whole. There are, however, certain bacterial poisons for which 
adequate antidotes are readily produced, thus, for example, for the 
diphtheria, tetanus, botulism, and possibly the dysentery poisons. 
Here the poisons can be more easily neutralized than the bacilli can 
be got rid of, but by neutralizing the poisons we succeed in arresting 
the multiplication of the bacteria and often in curing the disease. 
The normal body possesses a mean resistance to bacterial invasion 
and to bacterial poisoning which, while somewhat fluctuant, is of 
high value except under certain exceptional conditions in which 
infection readily develops. We know that certain general states of 
and influences exerted on the body are associated with a rise or a fall 
of this mean value. But we are not equally informed of the physical 
basis of this rise and fall. This particular topic is peculiarly difficult 
because of the large numbers of factors which enter into it. We know 
