REPORT ON THE CATTLE PLAGUE. 
97 
secondary causes are in full operation to predispose animals 
to its influence ; hence its continuance in the ill-ventilated, 
over-crowded, and badly-drained cow-sheds of the metropolis 
and other large towns, and on the <e cold retentive soils” and. 
undrained farms in the country, especially such as lie in ex- 
posed situations. 
Besides the special cause, or rather, perhaps, special com- 
bination of causes, which give origin to the enzootic form of 
pleuro-pneumonia, its appearance in a cattle-shed, or on a 
farm, is frequently traceable to the introduction of newly 
purchased nnimals, who bring the disease in a latent state 
with them; and which, on its declaring itself, extends by 
ordinary infection to those with whom they are located. 
Infection we hold to be one of the chief causes of the con- 
tinuance of pleuro-pneumonia for so many years among us, 
as every diseased animal, by virtue of the exhalations given 
off from its body, becomes a focus of the malady, and a new 
source, whence the poison, so to speak, is disseminated. The 
same fatality which marks the progress of pleuro-pneumonia 
here, attends it everywhere; and throughout the Continent 
it is looked upon as an incurable disease, and dealt with ac- 
cordingly. Its great fatality arises from the circumstance 
that the nature of the changes which take place in the lungs 
is such as immediately to arrest their functions as perfect 
aerifying organs, and soon to destroy, to a greater or less 
extent, the integrity of their structure. The true pathology 
of pleuro-pneumonia is among the quesiiones vexata of science. 
In this report we have not immediately to do with this ques- 
tion, still we may observe that the most eminent professors 
of veterinary medicine throughout Europe hesitate to declare, 
as some medical men have done, and others also who have 
probably given but little thought to the subject, that the 
changes wrought in the lungs are altogether due to inflam- 
matory action. 
In Belgium, in France, and in many parts of Italy, the 
disease is designated exudative pleuro-pneumonia— a name 
which, while it marks a peculiarity in the disease, implies, at 
the same time, that it differs somewhat in its results from 
ordinary inflammation of the lungs and their investing mem- 
brane, and which is correctly called pleuro-pneumonia. We 
have no hesitation in giving it as our opinion that the changes 
which are originally effected in the lung-tissue can take place 
otherwise than by inflammatory action. We observe, as the 
analogue of these changes, that in the advancement of the 
disease, the interstitial areolar tissue contiguous to the more 
affected parts of the organs, becomes primarily choked with 
