70 
R. KOCH. 
In like manner one can examine the most various objects by spreading out 
the substance to be examined as to its contents of tuberculous bacilli on the cov¬ 
ering-glass and by coloring the same. Nevertheless, one does not learn much 
more than that the bacilli are present in a tissue or in a, liquid and in what quan¬ 
tity they are present. Their position and their relations to the surrounding tis¬ 
sues cannot, in this way at least, be determined. The examination on the cover¬ 
ing-glass is therefore sufficient for liquids,* but for tissues can only have a pre¬ 
liminary, provisional character. Only the examination of the prepared sections 
of hardened parts can give reliable information as to the presence and diffusion of 
bacilli in the tuberculously altered organs. 
To find out whether bacilli are regular accompaniments of tuberculosis, I have 
examined as extensive a ground as possible. Materials for this investigation I 
have received for the most part from Dr. Friedlaender, who, at my request, and in 
the most obliging manner, made the rich material of the city hospital in Friedrichs- 
ham accessible to me, and from the director of the city hospital in Moabit, Dr. 
Guttman, who committed to my charge a number of cases of tuberculosis for ex¬ 
amination. It is a pleasant duty in this place to thank both gentlemen for the 
help they have given to my work. 
In the following description of the results gained in these investigations I must, 
in order to make a general survey, omit the historical enumeration of the single 
cases in the order in which accident placed them in my hand, and will speak of 
them as grouped according to the usual anatomic points of view. Before I turn 
to this, however, I must make a few general remarks. When a little tuberculous 
knot is examined in prepared sections, without the use of nucleus-coloring, and 
without the diffused light of Abbe’s illuminating apparatus, it appears like a body 
formed of cellular elements thickly crowded together and therefore only slightly 
transparent. As soon as the little tuberculous knot becomes caseous in the centre 
the cells change into a more or less fine grained, almost opaque mass, in which 
fine details are not to be distinguished. But a thoroughly different image of the 
tubercle is gained when the prepared sections are laid into strongly refractive 
media and the examination is undertaken after the nucleus-coloring, and by dif¬ 
fused illumination. The youngest tuberculous knots then show themselves to 
consist of colored grains heaped together. Nevertheless the grains are not so closely 
packed but that a section of ordinary thickness appears transparent enough to 
make it possible to distinguish the most delicate form elements occurring in the 
space between the grains. The caseous centres of the tuberculous knots in the 
prepared section appear wholly changed ; they appear almost uncolored and com¬ 
pletely transparent because there the cells have died and take no coloring; only 
here and there in them are found the remains of nuclei going to pieces, in the form 
of colored grain groups which, to be sure, are pretty closely pressed together but 
still allow all single form elements to be distinguished. Larger caseous herds 
conduct themselves in the same manner. The caseous substance itself has become 
completely transparent by the treatment and shows only a light greyish-yellow 
color tone interrupted by single brown grains or groups of grains. Every single 
tuberculous bacillus can be distinguished with ease. The conceptions of the mi¬ 
croscopic image of the tubercle and of the tuberculously altered tissue which usually 
obtain are to be modified according to the circumstances just described when the 
