Book I CONTAGION 35

they can even cause death in a few hours if they are analogous
with the spirits, but about this I shall say more later.  These
same germs can be shot forth from sore eyes into the eyes of
another and carry in a precisely similar infection, and this is
not a visual image but a taint in the eye. It is not surprising,
if one considers the method by which they attack, that they
penetrate into the animal, and some of them very quickly,
for they attack and enter from the small pores, veins and arter—
ies into the larger, and from these to others, and often reach
even to the heart.

One method of penetration is by propagation and, so to
speak, progeny. For the original germs which have adhered
to the neighboring humors with which they are analogous, gen—
erate and propagate other germs precisely like themselves, and
these in turn propagate others, until the whole mass and bulk
of humors is infected by them. A second method of penetra—
tion is by attraction, which works inwards, partly through
the breath by inspiration, partly by dilatation of the blood—
vessels. For along with the air that is drawn in, there enter,
mixed with it, germs of contagions, and when once these have
been introduced, they do not retire as easily by expiration
as they entered by inspiration; for they adhere closely to the
humors and organs, and some even to the spirits, which re—
treat from the image of their contrary, and carry their enemy
with them even to the heart. For we must not say, as some
do, that poisons and contagions try above all to make for and
attack the heart, like an enemy, as though they possessed cog—
nition and will.

Those that are per se less acute or are buried in great vis—
cosity, and are analogous with the denser humors, and are
drawn along by the blood—vessels, penetrate more slowly.  Those
that are introduced by the breath and are more volatile and
more acute, and are analogous with the spirits, penetrate more
quickly. 'There is perhaps yet another method of penetra—
tion.  For all evaporation spreads very easily from a narrow
space into one more ample; hence, since the blood—vessels near
the periphery are smaller and narrower, and then become larger
and larger as they approach the heart, the result is that the
contagion is very easily diffused from the narrow to the larger
blood—vessels, where also there is greater heat, and thence it
is carried even to the heart, unless there be some obstacle.