modern fixed ammunition is practically free from germs. The old
spear-head, the arrow, the cross bow bolt, had little regard for the
probabilities of infection. Whether infection follows a wound depends
both upon the entry of pathogenic organisms and upon these finding in
the tissues suitable opportunities for growth. In wounds in which
there is much laceration of tissue organisms find the most favorable
conditions for development. The very slight wounds produced by the
exploded cap in the toy pistol give suitable conditions for the
development of the bacillus which produces tetanus or lockjaw. The
deaths of children from lockjaw following a Fourth of July celebration
have often exceeded the total deaths in a Central American revolution.
The tetanus bacillus is a widely distributed organism, whose normal
habitat is in the soil and which is usually present on the dirty hands
of little boys. The toy-pistol wounds are made by small bits of paper
or metal being driven into the skin by the explosion of the cap. The
wound is of little moment, the surface becomes closed, and a bit of
foreign substance, a few dead cells and the tetanus bacilli from the
surface remain enclosed and in a few days the fatal disease develops.
Infection of the surfaces of old wounds such as the surface of an
ulcer takes place with difficulty. Large numbers of leucocytes which
give protection by phagocytosis are constantly passing to the surface,
and there is also a constant stream of fluid towards the surface. On
such a surface there may be an abundant growth of pathogenic
organisms, but no infection results.

In most infections there is a focus where the infectious organisms are
localized; this may correspond to the point of entrance on a surface
or it may be in the interior of the body, the organisms being
deposited there after entrance. At this primary localization, the
_atrium_ of infection,[1] the organisms multiply and from this