688 ON MURRAIN, OR THE VESICULAR EPIZOOTIC. 
progress, still remain to a great extent unexplained. The variety 
of circumstances in which the disease occurs, and the ever-changing 
influences which appear favourable to its development, present 
very serious difficulties in the way of etiological investigations. 
It appears inexplicable why a disease should exist and extend itself 
under the most opposite conditions ; yet such is most remarkably 
the case with this epizootic. It has spread alike in the coldest and 
warmest weather, in the wettest and driest situations. It has ex- 
isted in all localities, and has been but little affected by varieties 
of situation and exposure. Differences in feeding have made no 
difference as to its progress or severity. Animals in good condition 
and well tended have fallen victims to its attacks as well as those 
which were starved and neglected. It has appeared in the cleanest, 
driest, and best regulated byres, as well as in the most foul, damp, 
and badly ventilated hovels. Cattle at pasture have suffered from 
it equally with those in courts and byres. It has attacked indis- 
criminately animals of all ages, of all temperaments, and of both 
sexes. 
Since, therefore, the vesicular epizootic has existed and propa- 
gated itself in the midst of such numerous and even antagonistic 
circumstances, it is evident that the circumstances in themselves 
could have but little effect, or at most but a secondary effect, in 
producing or extending the disease. Indeed, no external circum- 
stances or influences, such as heat, cold, wet, drought, exposure to 
winds and weather, bad housing, want of ventilation, filth, neglect, 
starvation, or any sort of bad management are in themselves ade- 
quate to generate the disease. But although not one of these condi- 
tions, or even several of them put together, could produce vesicular 
epizootic, still they act in conjunction with the more immediate 
exciting causes, and induce, by the depression of the vital powers, 
a state of body favourable for the reception and development of 
the germ of the disease, and in so far they may be considered as 
remote or predisposing causes. 
These causes not only increase the susceptibility to receive the 
disease, but also augment its severity when it does occur. More- 
over, atmospheric changes, as electrical and meteorological influ- 
ences, from their powerful effect upon the animal organisation, may, 
also modify, to a certain extent, the nature of the disease, and may 
perhaps, have some effect in producing that wide difference observ- 
able in the intensity of the disease as occurring at various times 
and in various localities. 
The immediate or exciting causes of the vesicular epizootic are 
less numerous and perplexing than the remote or predisposing ones. 
Of the exciting causes, contagion appears to be the most active 
and effectual, To its influence the great majority of cases which 
