AETIOLOGY OF TUBERCULOSIS. 
155 
develop and form spores in a comparatively short time in a temperature correspond¬ 
ing to summer warmth. But this is not the case. The lowest limit of tempera¬ 
ture in which the tuberculous bacilli are able to growisnot reached by the summer 
temperature ; also, the growth of these bacteria goes on so slowly that they would 
be crowded out by the much more quickly developing sorts of bacteria everywhere 
appearing before they had finished their course of development. Even if, there¬ 
fore, other more easily obtainable substrata than those of an animal nature were 
found, which could serve the tuberculous bacilli as breeding ground, nevertheless 
the last mentioned reasons would speak decisively against the supposition that the 
tuberculous bacilli could lead an existence independent of the animal organism. 
We are, therefore, compelled, so far as our experience reaches, to consider the tu¬ 
berculous bacilli not as bacilli which can grow anywhere, but as genuine parasites, 
that is such as can find the conditions of their existence only in the animal or 
human organism. 
D.—ATTEMPTS AT INFECTION. 
These attempts until recently have formed the most important part of the 
experimental investigations concerning tuberculosis. But although these have 
been carried on in a very extensive manner, they lack, except in a few instances, 
the prudential measures which must necessarily be united with them to make them 
free from objection. 
There are three sources of error which can raise a doubt in regard to the 
attempt at infection. First, mistaking spontaneous tuberculosis for the tuberculosis 
artificially created by infection. Second, the mistaking of products of genuine tuber¬ 
culous disease for pathological changes, which with the naked eye or even microscop¬ 
ically, more or less resemble them. Third, unintentional infection with tuberculous 
virusby means of infected instruments, inoculating material, etc., in short through 
the neglect of antiseptic prudential measures. How shall one protect himself against 
these sources of error ? To avoid the errors arising from spontaneous tuberculosis, 
it has been suggested that one experiment only with those animals in which tuber¬ 
culosis seldom or never occurs. But since animals in which no spontaneous tu¬ 
berculosis occurs are always more or less indisposed to this disease, and therefore 
furnish no reliable reagent for the effect of the tuberculous virus, this proposition 
is practically not feasible. Also for attempts in anthrax infection one would, for 
example, not chose for exclusive use dogs, which, as is well known, are almost 
exempt from this disease; but, on the contrary, experiment with animals which 
are as sensitive as possible to anthrax-infection. The same holds of attempts at 
infection with tuberculosis. The more sensitive, therefore, a kind of animal is to 
infection with tuberculous virus, so much the better it is adapted to the infectious 
attempts in question. Nevertheless only under the condition that one succeeds 
in keeping the artificial and the spontaneous infection separate in the animals used 
for experiment. With some little attention this is not so difficult. The charac¬ 
teristic marks by which the two are to be distinguished have already been given 
in detail. It is self-evident, however, that, even though by means of these charac¬ 
teristics spontaneous tuberculosis be excluded as a cause of mistake, all prudential 
measures should be taken to confine the spontaneous disease to as narrow a field 
as possible. This may be attained by separation of the tuberculous animals in 
different cages, by frequeut airing, cleaning and disinfecting of the stalls. Never- 
