FISTULA. 
357 
isms may be regarded as specific exciters of suppurative tissue 
changes. 
Infection , Sources. — It is not difficult to enumerate 
sources from which infection may be received, but it is fre¬ 
quently impossible to prove a specific origin in any individual 
case. Pyogenic bacteria have a wide diffusion in nature, for 
they have been demonstrated in air, soil, water and a variety of 
foods. There is the old question of hereditary transmission, or 
transmitting living germs, and seems to be gaining ground with 
plenty of evidence in its favor. 
Modes of Entrance .—There are three ways in which micro¬ 
organisms usually penetrate into our bodies: i. From the 
surface of the skin, generally after it has been injured in 
some way. It does not, however, indeed always require such a 
special door of entry [i. e., abrasion). 2. The digestive canal, 
into which the bacteria pass along with the food; many, it is 
true, cannot pass through the stomach in their usual form, being 
destroyed by the action of its acid contents. Other kinds are 
less sensitive, and when spores are present, or when disease has 
altered the character of the digestive fluid and weakened its 
bacteria-killing power, there is no further obstacle to the pas¬ 
sage of the parasites. 3. The respiratory organs can afford 
entrance to the bacteria. 
Locating Factors. — After discussing the bacterial origin, 
sources from which infection may be drawn, and mode of en¬ 
trance, there remains the query, What determines the location 
of the pre-fistular abscess ? It has long been a matter of dis¬ 
pute whether pathogenic microbes exist in healthy animal 
tissues. I think the view is gaining general support that such 
conditions are possible, but that these same microbes do not 
exhibit their pathogenic properties sjo long as they remain in 
circulating blood and every body tissue remains healthy. Then 
to furnish the requisite conditions under which these pyogenic 
microbes may exhibit their pathogenic functions, some suscept¬ 
ible tissue must become injured or diseased. The blood, or 
lymphatic fluid, must be checked in its flow at that point, 
