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Page 34
Infection rarely takes place from the surface of the gullet or
oesophagus which leads from the mouth to the stomach. This is due to
the smoothness of the surface and to the rapidity with which food
passes over it. Infection by the stomach also is rare, for this
contains a strong acid secretion which destroys many of the bacteria
which are taken in with the food. It is found impossible to infect
animals with cholera unless the acidity of the stomach contents be
neutralized by an alkali. Many organisms, although their growth in the
stomach is inhibited, are not destroyed there and pass into the
intestines, where the conditions for infection are more favorable.
This large and very irregular surface is bathed in fluid which is a
good culture medium and but a single layer of cells covers it. The
organisms which cause many of the infectious diseases in both man and
animals find entrance by means of the alimentary canal, as cholera,
dysentery, typhoid fever, chicken cholera, hog cholera.
Infection by the genito-urinary surface is comparatively rare. The
surface openings are usually closed, and the discharge of urine has a
mechanical cleansing effect. The wide tube of the vagina is further
protected by a normal bacterial flora which produces conditions
hostile to other and pathogenic bacteria. The most common infections
are the sexual diseases, which are due to organisms which find
favorable conditions for growth in and on the surface and which are
conveyed from a similar surface by sexual contact.
It remains a question whether bacteria can penetrate an intact surface
producing no injury at the point of entrance and be carried by the
lymph or blood into internal organs where they produce disease.
Internal infections are often found with seemingly intact body
surfaces, but it is impossible to exclude the presence of minute or
microscopic surface injuries by which the organisms may have entered.
It is also possible that a slight injury at the point of entrance may
heal so completely as to leave no trace.
The chief danger from wounds is that their surfaces may become
infected. Death from wounds is due more frequently to infection than
to the actual injury represented by the wounds. Much depends upon the
character of the wound. Infection of clean wounds which are made by a
sharp cutting instrument and from which there is abundant h�morrhage
with sealing of the edges of the wound by clotted blood, rarely
happens. Typical wounds of this sort are often made in shaving, and
infection of such wounds is extraordinarily rare. If, with the wound,
pathogenic organisms are placed in the tissue, or foreign substances
such as bits of clothing are carried in with a bullet, for example, or
if the instrument causing the wound be of such a character as to
produce extensive lacerations of tissue, infection is more apt to
occur. The less frequency of infection in modern wars is in part due
to the simpler character of the wounds and in part to the fact that
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
point further invasion takes place. Many secondary foci may be formed
in the organs by distribution of the organisms, or there may be
infection of the blood and fluids of the body. The injuries which are
produced depend upon the nature of the infecting organisms. The most
common lesion consists in the death of the tissue about the infecting
organisms. In most cases the sum of the changes are so characteristic
that from them the nature of the infection is easily determined, and
these changes often give names to the disease; thus tuberculosis is a
disease characterized by the formation of tubercles or little nodules
in the body. The situation of the foci of disease is determined by
many conditions, the most important being the varying resistance of
the different organs of the body to the growth of bacteria. Certain
organs, such as the central nervous system, the muscles, the testicles
and the ovaries, have a high resistance to the growth of bacteria. The
disease may be localized in certain organs because only in these do
the bacteria find favorable conditions for growth. In spite of a high
general resistance to infection the lesions in chronic glanders are
most marked in the muscles, those of poliomyelitis in the spinal cord.
There are few bacterial diseases which are localized in the blood, but
many of the diseases caused by protozoa have this localization. In
every infection some organisms enter the blood, which acts as a
carrier and deposits them in the organs.
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