AETIOLOGY OF TUBERCULOSIS. 
203 
an opinion now as to the relations of tuberculous bacilli to the disease caused 
by them. 
If we start from the experimentally proved proposition that only tuber¬ 
culous bacilli have the power of generating genuine tuberculosis, and if we 
apply ourselves to following the way which the bacilli take in the infection, 
the question of the origin of the bacilli first forces itself upon us. Do they 
occur anywdiere, and independent of the human or animal organism, in the 
outer world, as for example must be concluded of the inflammation of the 
spleen bacilli and the micrococci of erysipelas ? The answer to this question 
is of the greatest importance, not only for the astiology, but also much more 
for the prophylaxis. For, granted the tuberculous bacilli live in the decay¬ 
ing animal or vegetable materials everywhere to be found, that they can in¬ 
crease and form spores, then it would hardly be possible to keep these para¬ 
sites away from man. But fortunately it is otherwise. Experience has 
taught that the tuberculous bacilli grow much more slowly than all other 
bacteria ; further, that they only grow in blood serum and meat liquid, and, 
which is the main point, they need temperatures of more than 30° C. in order 
to thrive. Also, when all these conditions were found united, but the tuber¬ 
culous bacilli were not protected against the luxuriant growth of other 
quickly growing bacteria, then, as one can often enough see in the cultures 
corrupted by foreign bacteria, the bacilli would be crowded out and killed 
by the rival bacteria. Now, indeed, the conditions of development favorable 
for tuberculous bacilli, especially the warmth of 30° C. day and night for 
weeks are found united nowhere except in the animal organism, and there is, 
therefore, no other supposition possible than that they are dependent for their 
existence wholly upon the animal and human organisms. They are there¬ 
fore, genuine parasites, which cannot exist without a body to support them. 
They are not like the anthrax-bacilli, accidental parasites, which usually com¬ 
plete their course of development in the outer world and only occasionally 
make an invasion into the animal body. There exists also an essential differ¬ 
ence between the anthrax-bacilli and the tuberculous bacilli, in that the first 
only multiply in the animal body, but never form spores, and for the develop¬ 
ment into the permanent form must get into the outer world again, while the 
tuberculous bacilli complete their entire course of development in the body 
and in no way need a life in the outer world in order to take the form neces¬ 
sary for the preservation of the species. 
Another question is, whether from the wide-spread bacteria, which often 
get into the body, under favorable conditions, by means of adaptation and 
successive breedings, tuberculous bacilli might not arise, or on the other hand 
the tuberculous bacilli either in the body or after they had left the same 
might not change into harmless bacteria. It would then not need the inva¬ 
sion of specific bacteria to develop tuberculosis, but all would depend on the 
necessary preparatory conditions for changing harmless into harmful bac¬ 
teria, which would be all the same as one usually calls tendency. The rep¬ 
resentation of a cross-breeding of tuberculous bacilli corresponds exactly with 
the now often held but widely exaggerated views of the changeable nature 
of bacteria, and has already found supporters. More value than that of a 
