ON MURRAIN, OR THE VESICULAR EPIZOOTIC. 689 
have occurred in this country may be traced. The fact of a few 
instances of the disease having sprung up independently of conta- 
gion, is no argument against its general diffusion by contagion. 
Such a phenomenon is observable in many epidemic and epizootic 
diseases, of whose contagion no doubt can be entertained. It has 
frequently occurred in the progress of small-pox; and, as Dr. 
Watson remarks, “ If small-pox be produced by contagion alone, 
and yet the mode in which the contagious matter has been com- 
municated eludes sometimes our closest scrutiny, then we must 
conclude that the same thing may happen in other contagious dis- 
eases, of which the contagious property may not be so strong or so 
obvious.” And this ought surely “ to warn us,” says the same 
author, “ against the inferring of analogous disorders that they are 
necessarily not contagious, because we often fail to discover any 
way in which the poison could have been applied.” 
As an argument against the contagion of the vesicular epizootic, 
it has been asserted that animals, placed in circumstances ap- 
parently the most favourable for the production of the disease, 
have sometimes altogether, or for a time, escaped its attacks, while 
the rest of the stock have been affected. It has been further as- 
serted, that repeated inoculations have failed to produce the dis- 
ease in sound cattle and sheep. 
But these arguments only prove that the disease is more readily 
generated in some states of the constitution than in others. The 
indisposition to receive and mature the virus, whether communi- 
cated by contagion or more directly by inoculation, may in many 
instances have resulted from the animals having previously been 
affected by the disease. This immunity from a second attack is 
explained, according to Professor Liebig’s ingenious theory of the 
propagation of contagious disorders, by the supposition that the 
occurrence of any such disease exhausts and destroys for a time 
that principle in the blood, which he believes to be essential for 
the reproduction of the virus and the existence of the disease. 
The strongest evidence in favour of the contagious nature of the 
vesicular epizootic, is derived from its history and progress ; its 
slow and gradual spread ; its appearence among a stock or in a 
district being in very many instances traceable to communication 
with or the introduction of diseased animals; its affecting animals 
of all ages and under all circumstances ; its frequent occurrence in 
and return to the same neighbourhood, and at the time of large 
fairs; its scarcely ever having existed in certain localities, and in 
certain farms, where strict and efficient means were adopted for 
keeping the stock out of the way of contagion. Besides these 
sources of evidence, numerous well authenticated and incontro- 
vertible facts, derived from the observation of many cases, might 
