Tlie Tubercle Bacillus. 
85 
The mere disoovery of a particular parasite in association 
with a disease is, however, no proof that it is the cause of the 
disease. To establish a causal relation a great deal of evidence 
is necessary, for it must be proved that the micro-organism does 
produce the disease in question. To obtain such evidence we must 
first isolate the bacillus from tubercular tissues ; it must be grown 
artificially outside the animal body, and with a pure cultivation 
we must be able to produce the disease by introducing it into 
susceptible animals. 
The Cultivation of the Tubercle Bacillus. 
Koch pointed out that, if a fragment of tissue containing the 
bacillus were inserted into solidified blood serum ' a growth of the 
"bacillus took place, which after a few weeks became easily visible 
to the naked eye. Minute dry scales developed on the surface of 
the cultivating medium. By sowing the bacillus in fresh tubes 
or flasks containing this medium, the cultivation of the bacillus 
could be kept up indefinitely. The cultivating medium, what- 
ever its composition, nmst at the outset be free from all micro- 
organisms of every kind. 
For most bacteria, beef-tea containing gelatine is used. A 
small quantity is allowed to set in a test-tube, the most convenient 
vessel for use in the laboratory. This medium readily liquefies 
if the temperature is raised ; and if, therefore, it is necessary to 
grow the bacteria in an incubator, a meat-jelly is used in which 
Japanese isinglass (agar-agar) is substituted for the gelatine. 
After sowing the bacteria on the surface of this jelly, the test- 
tubes are placed in the incubator at the temperature of the 
blood. 
For the cultivation of the tubercle bacillus Koch, as already 
mentioned, employed solidified sterilised blood serum, but the 
preparation of this medium took so much time and was so com- 
plicated that but few succeeded in verifying his discovery. 
On the other hand, the preparation of meat-jelly is compara- 
tively easy. A great advance was therefore made in the study 
of the life-history of the tubercle bacillus when the French 
bacteriologists, Nocard and Roux, discovered that the addition 
of 5 per cent, of glycerine to meat-jelly prepared with agar-agar 
afforded a soil on which the tubercle bacillus could be cultivated 
in abundance with the greatest facility. 
' Blood is allowed to clot in clean glass vessels, and the liquid part, or 
sorum, is decanted into test-tubes and solidified by heat. 
