tissue are placed in sterile
vessels and offered to the animals.

FOOTNOTES:

[12] This table is made by Messrs. Down Bros., St. Thomas's Street,
London, S. E.

[13] This modification is made for the author by Messrs. Down Bros., St.
Thomas's Street, London, S. E.

[14] Manufactured by Messrs. Francis Lepper, 56, Great Marlborough
Street, London, W.

XVIII. THE STUDY OF EXPERIMENTAL INFECTIONS DURING LIFE.

The possession of pathogenetic properties by an organism under study is
indicated by the "infection" of the experimental animal--a term which is
employed to summarise the condition resulting from the successful
invasion of the tissues of the experimental animal by the
micro-organisms inoculated and by their multiplication therein.
Infection is considered to have taken place:

1. When the death of the animal is produced as a direct consequence of
the inoculation.

2. When without necessarily producing death the inoculation causes local
or general changes of a pathological character.

3. When either with or without death, or local or general changes
occurring, certain substances make their appearance in the body fluids,
which can be shown (_in vitro_ or _in vivo_) to exert some profound and
specific effect when brought into contact with subcultivations of the
organism originally inoculated.

The important factors in the production of infection are:

A. Seed. Virulence of organism.
Dose of organism.

B. Soil. Resistance offered by the cells of the experimental animal.

The first two factors, although variable, are to a certain extent under
the control of the experimenter. Thus by suitable means the virulence of
an organism can be exalted or attenuated, whilst the size of the dose
may be increased or diminished. The third factor also varies, not only
amongst different species of animals, but also amongst different
individuals of the same species. The essential causes of this variation
are not so obvious, so that beyond selecting the animals intended for
similar experiments with regard to such points as age, size or sex, but
little can be done to standardise cell resistance.

Immediately an animal has been inoculated a period of clinical
observation must be entered upon, which should only terminate with the
death of the animal. The general observations should at first and if the
infection is an acute one, be made daily--later, and if the animal
appears to be unaffected or if the inf