Pleuro- Pneumonia amongst Cattle. 
359 
may be, comes nevertheless within that class of diseases termed 
Epizootic. 
The subject of Epizootic diseases is one of an extensive range, 
and admits of a wide field for speculation ; there are, however, 
some general as well as particular circumstances which we should 
not be justified in overlooking. The manner in which this, or 
diseases of a similar kind, are not only observed to prevail over 
extensive districts of the same country, but also in different 
countries themselves, would lead us to ascribe their origin, as well 
as their specific character, to peculiar atmospheric conditions ; 
but as yet so little has been positively ascertained regarding those 
conditions, that whilst we are obliged to take them into account, 
whilst dealing with the general history of the disease, we are not 
able to draw any practical application from them. I may how- 
ever mention that, unusual and irregular weather — the preva- 
lence of particular winds — varieties of season, as drought and 
moisture, may exercise some influence on the development of 
the disease : but what is of more practical importance, that 
local conditions, either as regards situation of pasturage and 
quality of herbage — impurities of stabling — stall-food, either 
defective in quantity or quality, may also materially contribute to 
its production. 
And in addition to the above mentioned external causes, I 
must not fail to call attention to those conditions in which foreign 
cattle are placed before they arrive in this country — such as a 
voyage by sea — confinement on deck, &c. ; or the manner in 
which they or home-bred cattle are driven from one place to an- 
other, under which circumstances they are made to undergo much 
fatigue, and are generally obliged to pick up food by the road- 
side, or in limited pastures ; being at the same time exposed 
to all varieties of the weather. However, many of the incon- 
veniences connected with the forwarding of cattle from one 
place to another, are now obviated by the facilities afforded by 
railways. 
If we take a general survey of the causes just brought forward, 
we shall find that they almost collectively lead us to the conclu- 
sion that the manner in which cattle are met with, congregated 
together at fairs, markets, or passed from place to place, favours, if 
not the production, yet in a most remarkable manner the dissemi- 
nation of the disease. 
It is true that we might at first sight ascribe the frequent oc- 
currence of the disease in cattle so circumstanced to infection, but 
we must bear in mind that they have all been placed in the same 
conditions, and that there can be no more reason that one should 
escape the effects produced by such conditions than another. We 
must, however, give some restriction to the last expression, be- 
