to resist the attacks of the phagocytes. The method is most successful
when the vaccine is prepared from organisms isolated from the patient
himself, _autogenous vaccine_, but when this is impracticable, or takes
a considerable time, laboratory-prepared polyvalent _stock vaccines_ may
be used.

_Clinical Use of Vaccines._--Vaccines should not be given while a
patient is in a negative phase, as a certain amount of the opsonin in
the blood is used up in neutralising the substances injected, and this
may reduce the opsonic index to such an extent that the vaccines
themselves become dangerous. As a rule, the propriety of using a vaccine
can be determined from the general condition of the patient. The initial
dose should always be a small one, particularly if the disease is acute,
and the subsequent dosage will be regulated by the effect produced. If
marked constitutional disturbance with rise of temperature follows the
use of a vaccine, it indicates a negative phase, and calls for a
diminution in the next dose. If, on the other hand, the local as well as
the general condition of the patient improves after the injection, it
indicates a positive phase, and the original dose may be repeated or
even increased. Vaccines are best introduced subcutaneously, a part
being selected which is not liable to pressure, as there is sometimes
considerable local reaction. Repeated doses may be necessary at
intervals of a few days.

The vaccine treatment has been successfully employed in various
tuberculous lesions, in pyogenic infections such as acne, boils,
sycosis, streptococcal, pneumococcal, and gonococcal conditions, in
infections of the accessory air sinuses, and in other diseases caused by
bacteria.

PYOGENIC BACTERIA

From the point of view of the surgeon the most important varieties of
micro-organisms are those that cause inflammation and suppuration--the
_pyogenic bacteria_. This group includes a great many species, and these
are so widely distributed that they are to be met with under all
conditions of everyday life.

The nature of the inflammatory and suppurative processes will be
considered in detail later; suffice it here to say that they are brought
about by the action of one or other of the organisms that we have now to
consider.

It is found that the _staphylococci_, which cluster into groups, tend to
produce localised lesions; while the chain-forms--_streptococci_--give
rise to diffuse, spreading conditions. Many