90 
The Tubercle Bacillus. 
Microscopical Examixatiox of the Bacilli. 
The bacilli are colourless, and therefore not easily distin- 
guished from the tissues in -R-hich they grow ; we have, there- 
fore, to employ the process of staining them with an aniline dye. 
Bv this means we can readilv diflferentiate the bacilli, and studv 
their form and their relation to the tissues in which they grow. 
It is this method which gives us an invaluable means of ascer- 
taining the existence of tuberculosis. 
To illustrate this, let us suppose that we have to deal ^vith 
an indurated condition of the udder of a cow. To distinguish 
between chronic inflammation and tubercular disease may be a 
matter of the greatest difficulty, and perhaps of impossibility, if 
we trust to symptoms and physical signs. And yet it is a matter 
of the greatest importance, for the danger of employing the milk 
of cows suliering from udder tuberculosis is. especially to children, 
a matter beyond doubt. 
But if some of the milk is drawn off into a test-tube and 
set aside for an hour, we can readily examine the sediment for 
tubercle bacilli. The supernatant liquid is poured off, and a 
little of the sediment spread out upon a cover-glass. This is 
allowed to dry and is fixed by passing the cover-glass through the 
flame of a gas-burner or a spirit-lamp. The cover-glass is then 
floated, ^vith the prepared side downwards, on hot carbolised 
fuchsLne ^ in a watch-glass for three to five minutes. The cover- 
glass is next transferred to dilute sulphuric acid, which washes 
the stain out of eveiything except the bacilli. If desired, a 
contrast stain can be given to the rest of the preparation by 
placing the cover-glass for a couple of minutes in methylene 
blue. In such preparations the tubercle bacillus is stained 
brilliantly red, and the tissue cells and any other micro-organ- 
isms are stained blue. 
In dealing with the diseases of animals, this power of re- 
taining the stain in the presence of acid is an absolute indica- 
tion of the presence of the tubercle bacillus. In man this 
statement does not hold good, for the same occurs in the case of 
the closeh" allied leprosy bacillus. 
If now the cultivations of the bacillus are examined by the 
same method, we can readily detect the same bacilli ; but they 
are found to Ije shorter and thicker than in human sputum, and 
for the most part without the beaded appearance. In old 
' A crimson aniline dye prepare<l by dissolving 5 parts of crystals of fuchsine 
in 10 parts of alcohol and 100 parts of a 5 per cent, watery solution of carbolic 
acid. 
