56 
R. KOCH. 
mistaking artificial tuberculosis for that arising spontaneously is also completely 
excluded. Therefore in comparing the attempts of Cohnheim and Salomonsen 
with those previously made, they must be admitted to be completely free from* 
objection, and by them it is proved that the most different tuberculous substances 
contain a specific and individual element. 
Of what character this infectious matter might be, whether it be formed by 
means of organisms independent, or provided with unvarying properties, which 
force themselves into the body as parasites and communicate tuberculosis to it, or 
whether the infectious material exists only in certain conditions of the body and 
consists of organized or unorganized formations arising from its own ingredients, 
was a question that for the time being could not be decided. But after the results, 
which have been attained of late years regarding the (etiology of many infectious 
diseases, the possibility that also the cause of tuberculosis was to be sought in 
some sort of micro-organisms, seems to be indicated. In order to obtain light on 
this point, it was necessary to make use of all that experience which had proved 
itself useful in the examination of other infectious diseases, and the same course 
of investigation was to be chosen which in other cases had shown itself most to 
the purpose. To make the best use of these advantages, the attempts were to be 
made in the following manner: In the first place, it must be decided whether form 
elements, not belonging to the elements of the body, or arising from them, are 
present in the diseased parts. If the existence of such can be proved, then we 
must investigate whether the same be organized and whether they offer any signs 
of independent life, among which independent motion, (for which molecular mo¬ 
tion is very often mistaken), growth, increase and generation are to be reckoned. 
Further, the relations to their surroundings, the conduct of the components of 
the neighboring textures, their diffusion in the body, their appearance at the va¬ 
rious stages of the disease, and other circumstances must be traced, which, with 
more or less probability, show a primary connection between these formations 
and the disease. It is possible that the facts gained in this manner may afford 
such proof that only the most extreme scepticism can maintain that micro-organ¬ 
isms found are no cause, but only an accompaniment of the disease. Often, of 
course, this objection is justified, and therefore it is necessary for a complete 
system of proof that one should not confine himself to showing the co-existence 
of the disease and parasites, but ’ that, more than this, these parasites must be 
shown to be the real cause of the disease. This can only be done by removing 
the parasites from the diseased organism completely, and freeing them from all 
products of the disease to which a hurtful influence could be ascribed, and that 
by the introduction of the isolated parasites into the healthy organism, the dis¬ 
ease with all its peculiar characteristics should be produced anew. An example 
may serve to explain what has just been said. If the blood of animals which 
have died from inflammation of the spleen be examined, there are always 
found therein numerous minute, rod-shaped, colorless, motionless forms. One 
could not see immediately that these minute rods are of a vegetable nature, and 
indeed in the beginning they were often held for lifeless crystalline bodies. First 
from the circumstance that they were seen to grow from spores, and that from 
the spores little rods were developed anew, could it be decided with certainty 
that they possessed life and belonged to the lowest class of plants. Further, 
