16 
EPIZOOTIC PLEURO- PNEUMONIA. 
of cattle, particularly Irish, are brought to this market ; in the 
former for the straw-yard, and in the latter for summer feeding in 
our marshy districts. They are brought here in large droves, and 
frequently several lets of cattle of different breeds belong to the 
same dealer. Although they are separated while in the market, 
they are frequently drifted and pastured altogether : hence cattle of 
every breed have become infected. 
I have intentionally omitted making any remark as to the nature 
or dryness, or moisture of the different situations. In my opinion, 
neither the soil nor the locality have much influence in promoting or 
controlling the virulence of this disease. Its greater prevalence in 
our fields is clearly in consequence of farmers and graziers buying 
cattle at the spring fairs and out of the markets, and sending them 
to these places for summer grazing, &c. Again ; the dairy cows 
being so frequently infected is attributable, in many cases, to the 
sick having been placed in yards or houses adjoining theirs. 
Causes . — The principal causes of the spread of pleuro-pneu- 
monia are, first, infection ; secondly, this disease not making a 
decided appearance until several weeks after the period of infec- 
tion ; thirdly, almost every person, as soon as he is aware that any 
of his cattle are infected, disposes of them : and, in the majority of 
instances, by sending them to a cattle fair or market, where they 
are frequently sold to farmers, who, being ignorant of their hav- 
ing been infected, or having been with infected cattle, take no 
precaution ; and thus this disease is frequently communicated to 
other stock. 
Infection . — The words infection and contagion have been re- 
ceived by some writers as altogether synonymous, while others 
have drawn distinctions between them. Few, however, of the 
latter have agreed ; in fact, authorities on the subject are contra- 
dictory, and are more calculated to perplex than to guide. In the 
present case I shall employ the word infection, it being, in my 
opinion, more applicable than the latter term. By the word infec- 
tion, then, is meant the transmission of disease from the infected 
to the healthy beast, by a morbid miasm or exhalation being dif- 
fused in the air, the emanations with which the air is charged act- 
ing on animals in the same manner as poisonous agents. 
The principal infecting agents consist of diseased organized 
substances, and of animal exhalations. The interesting researches 
of M. Magendi, in order to ascertain the effects of putrid animal 
matter, shew that dogs confined over and breathing the effluvia 
proceeding from animal substances in a state of decay, • produce 
disease of a low typhoid character; and that, after death, the 
blood was found remarkably altered, being nearly or altogether 
composed of fluids of a very dark colour, and partially exuded 
