86 
Tlie Tubercle Bacillus. 
Just as we can grow tropical plants in tliis country by 
supplying the necessaiy conditions of temperature, moisture, 
and soil in hothouses, so these microscopic plants or bacilli 
can be cultivated outside the animal body by supplying the 
necessaiy food and by keeping them in an incubator with careful 
regulation of the temperature. 
It is not absolutely necessary to employ the cultivating 
media in a solid state, but it is preferable when we require to 
study the appearance as presented to the naked eye of the bacilli 
en masse. Beef-tea with glycerine, but without either gelatine 
or agar-agar, can be employed, and when the growth has 
developed to its full extent, the bacilli can be separated by 
filtration through porcelain, and the filtrate examined. The 
secretion of the bacillus, if such has been formed, is thus 
separated, and the changes which have taken place in the liquid 
by the digestion of the complex substances in the broth can be 
ascertained. 
Tubercle Bacilli and the AVhite Blood Corpuscles. 
Having obtained cultivations on either solid or liquid media, 
the next step is to inoculate susceptible animals. Koch assured 
us that the inoculation with the isolated bacillus of such animals 
as rabbits, guinea-pigs, and monkeys, produced the disease tuber- 
culosis, and hence the tubercle bacillus is very generally accepted 
as the active agent in the production of this disease. 
The hereditary character of consumption, however, indicates 
that we have to deal with something more than the life-history 
of the tubercle bacillus. If the disease be solely due to the 
inhalation of a micro-parasite, why should the disease be 
hereditary, and why should tuberculosis in cattle be increased'by 
breeding in and in ? We have obviously not only to deal with 
the parasite, but with other factors. Susceptibility to the disease 
can be increased by unhealthy surroundings and inherited. 
According to the theory of Metschnikoff, a Russian bacterio- 
logist, there is a constant warfare between the white blood 
cells ' and bacteria. If an animal has immunity from or is proof 
against a disease, and also in cases of recovery from an attack 
of a disease, the white blood cells or " phagocytes" attack and 
' The blood consists of a clear fluid called liquor tanffuinu, with little disc- 
shaped bodies called red corpuscles, and colourless protoplasmic cells called 
white corpuscles. The latter constantly change their form by sending out and 
retracting delicate processes like the small unicellular organisms known as 
amoeb.T. They migrate when a tissue is in need of repair, passing through 
the delicate wall of the minutest Mood vessels. 
