ed metal box and store in the ice chest for
future use.

FOOTNOTES:

[10] The quantities here given are not absolutely correct. If exactitude
is essential the student must calculate the amount required by the aid
of the Percentage Formula, Appendix, page 496.

[11] See Percentage Formula, Appendix, page 496.

XVII. EXPERIMENTAL INOCULATION OF ANIMALS.

The use of living animals for inoculation experiments may become a
necessary procedure in the Bacteriological Laboratory for some one or
more of the following reasons:

A. ~Determination of Pathogenetic Properties of Bacteria already Isolated
in Pure Culture~ (see page 315).

The exact study of the conditions influencing the virulence (including
its maintenance, exaltation and attenuation) of an organism, and precise
observations upon the pathogenic effects produced by its entrance into,
and multiplication within the body tissues can obviously only be carried
out by means of experimental inoculation; whilst many points relating to
vitality, longevity, etc., can be most readily elucidated by such
experiments.

B. ~Isolation of Pathogenetic Bacteria.~

Certain highly parasitic bacteria (which grow with difficulty upon the
artificial media of the laboratory) can only be isolated with
considerable difficulty from associated saprophytic bacteria when
cultural methods alone are employed; but if the mixture of parasite and
saprophytes is injected into an animal susceptible to the action of the
former, the pathogenic organism can readily be isolated from the tissues
of the infected animal. The pneumococcus for example occurs in the
sputum of patients suffering from acute lobar pneumonia, but usually in
association with various saprophytes derived from the mouth and pharynx.
The optimum medium for the growth of the pneumococcus, blood agar, is
also an excellent pabulum for the saprophytes of the mouth, and plate
cultures are rapidly overgrown by them to the destruction of the more
delicate pneumococcus. But inoculate some of the sputum under the skin
of a mouse and three or four days later the pneumococcus will have
entered the blood stream (leaving the saprophytes at the seat of
inoculation) and killed the animal. Cultivations made at the post-mortem
(see page 398) from the mouse's heart blood will yield a pure growth of
the pneumococcus.

C. ~Identification of Pathogenetic Bacteria.~

The resemblances, morphological and cultural, existing between certain
p