72 
R. KOCH. 
are in process of dying. Their nuclei begin to decompose and to change themselves 
into irregularly formed grains of very varying size. Gradually these become scarcer 
and there remains a uniform mass which will not take nucleus coloring and in 
which all the cells originally present have died. This mass forms what was 
formerly considered the essential part of the tubercle, as the bearer of the infectious 
material, namely, the caseous centre of the same. But, as a rule, this caseous 
substance is very poor in tuberculous bacilli. Only when the death of the 
cells and their change into the nucleusless caseous mass has taken place very 
quickly are the bacilli visible for a time in considerable numbers. It is plain that 
they retain the capacity of fixing the coloring matter longer than the cells perish¬ 
ing under their influence. But very soon the bacilli themselves undergo farther 
changes, either dying or go into the stage of spore formation, in which they gradu¬ 
ally lose their power of taking color. In the last case only their spores remain in 
the caseous substance, and as until now no means have been found of coloring the 
spores of tuberculi in any way whatever, their presence after the vanishing of the 
tuberculi betrays itself only by the infectious qualities of the caseous substance in 
which they are imbedded. On account of the importance, formerly and even very 
lately, erroneously attached to the caseous products of the tuberculous process, it 
may not be superfluous emphatically to direct attention to the facts that in all 
tuberculous affections the tuberculi appear first, collections of cells joining them¬ 
selves to these ; and that the dying of these cells and the caseous change resulting 
from this are secondary processes.* 
The opinion which still, to a great extent, holds ground that the relation 
between the bacilli and the caseous degeneration is the opposite of this, that the 
becoming caseous represent the primary, and that by means of this a suita¬ 
ble breeding ground is prepared for the tuberculous bacilli, is therefore com¬ 
pletely erroneous. For the anatomical comprehension of the tissue changes in 
consequence of tuberculosis the process of becoming caseous may be of interest, 
but for the aetiology of tuberculosis it has not the slightest importance. 
If I have lately been charged with paying too little attention to the process 
of caseous degeneration in my account of the aetiology of tuberculosis, the 
charge is unfounded, for it rests upon a misunderstanding of my standpoint, since 
I have only treated the aetiological relations of tuberculosis, but have left the 
pathological details to the pathological anatomists, especially when they lie so 
far aside from aetiology as the caseous changes of the tuberculous tissue. 
Of greater importance for the questions interesting us here are the relations 
of tuberculous bacilli to the gigantic cells so frequently appearing in tubercu- 
lously altered tissues. 
These peculiar formations are so frequent in tuberculous tissues that it was 
for a time believed that they must be considered as characteristic of tuberculosis. 
Since the gigantic cells are almost always situated at the centre of the little tuber¬ 
culous knot, the opinion has often been expressed that the tuberculous virus 
must be contained in their interior —has indeed been pointed out in the shape 
of very small grains. 
* Baumgarten, “Ueber die Wege der tuberkiilosen Infection. ” Zeitschrift f. klin. Med. 
Bd. VI, beft. I. 
