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the north-east, is a sufficient reason why we cannot look for an extension of
the widening circle in those directions, but wherever there may be population,
and an absence of hilly barriers to obstruct the circular extension of the cholera
wave, it will continue to spread out, until it be lost in the extreme east of China
on one hand, and Africa, Europe, and America to the west.
The cholera epidemic when once in motion, and with climatic conditions
favourable to its increase, goes on widening, year after year, for the period of its
natural life. The circle of the first year may be only a few hundred miles in
diameter, but like the " fairy rings" in our meadows, it continues to grow and
advance from its outer edge wherever no physical obstacles intervene.
And in this outward extension we shall find that uninhabited mountain tracts,
or mountain tracts, the people of which hold little intercourse with the low country,
also deserts, and seas, interfere with the regular expansion of the circle, while
the points of protuberance of the advancing wave will be indicated by the valleys
of great rivers, or tracts of country but little elevated above sea level.
197. It is to the fact of radiation of cholera, as from a centre, that we must
explain its appearance, and progress in opposite directions, at the same moment
of time in the Punjaub and Madras. In 1869 we know that cholera travelled
north-west into Cabul at the very moment it was extending southward in the
direction of Cape Comorin, and while travelling south in the Indian Peninsula, it
was moving south-east towards China, and westward through the African Conti-
nent. So far as we know any thing of the history of cholera in Africa, it has
never yet surmounted the mountain chain of the lake districts in the interior, and
its ravages have been expended on the eastern side of that mountain barrier.
198. The true explanation of this tendency of cholera to extend outwards
from its endemic habitat, I do not pretend to give. If I state my conviction that it
finds the elements of its growth and renewal on the outer edge of the widening
circle, I am merely repeating what everybody admits to be true, that cholera
prefers to attack a locality which has been long exempted, and in which the
population may be favourably disposed to receive and propagate the contagium.
199. What the law of susceptibility may have to say to this outward progress
of cholera it is hard to say, inasmuch as we know nothing about such a law,
though, practically in regard to some kindred diseases, we are content to
acknowledge that people are the more liable to suffer an incursion, who have been,
for a certain number of years, practically exempt from epidemic visitations.
And the same remark holds good of cholera ; when a district is newly in va ded,
it does not affect every spot of ground in the year of invasion ; but should it
outlive that year, the probabilities are in favour of the tracts exempted in the
first year suffering in the next, or following season, so that the liability of a
population to suffer from cholera, other things being equal, seems to bear some
proportion to the previous period of immunity. And I think we must also be
prepared to admit that the prior occupation of a locality by a wave of malaria
or small-pox will, for the time being, have the effect of preserving the inhabi-
tants of that locality from the cholera influence.
The effect of previous immunity seems well illustrated in regard to the
Godavery and Kistna Districts, which entirely escaped epidemic invasion by
cholera at the time when the invasion was normally due in 1869, but which
districts, when the seeds of the disease were imported in March 1870, were
quite ready to yield an abundant crop. The Godavery District suffered so