826 
PLEUROPNEUMONIA. 
true that the extension of the disease is not affected through 
the ordinary channels. It yet remains to be decided by what 
means the contagium is carried ; but at present the presump- 
tion is that the virus is given off by the lungs in such a form 
that it contaminates the atmosphere, and the more rapid 
spread of the malady in close sheds, as compared with its 
progress among cattle in the open country is a fact which 
supports this view of the matter. 
Period of Incubation of Pleuropneumonia. 
By the term incubation, in reference to an infectious dis- 
ease, is meant the time during which it remains in the system 
without causing any external manifestation. The period of 
incubation, therefore, is the interval which elapses between 
the reception of the poison and occurrence of illness. This 
time of “hatching^ may be determined with certainty in 
nearly all infectious disease, by inoculating a healthy animal 
with the poison of the malady, and carefully watching for the 
declaration of the first signs of illness, but in the case of 
pleuropneumonia the same test cannot be applied, because 
we have no means of inducing the disease with certainty ex- 
cept by exposure of the animal to infection through the 
medium of association with diseased cattle, a very unsatis- 
factory method of inquiry, because it fails to furnish the 
inquirer with exact evidence as to the time of infection. 
Notwithstanding the fact of cohabitation, healthy animals 
may resist the mal-influence of the exhalations from diseased 
beasts for weeks, while others may become susceptible to its 
influence in a few days, and the uncertainty as to the time 
of infection prevents the determination of the period of in- 
cubation. 
The shortest period which elapses between the exposure of 
the healthy animals to contact with diseased beasts and the 
appearance of the first symptoms of illness may be fairly taken 
as the actual time of incubation. The long intervals which 
very frequently occur between exposure to infection and the 
development of disease may be presumed to depend in many 
instances upon non-susceptibility of the animal exposed. In 
the experiments of the French Commission some of the 
animals which were placed in contact with the diseased cattle 
were the subjects of a peculiar cough in six days after expo- 
sure ; soon after, other signs of pleuropneumonia appeared, 
and from the sixteenth to the fiftieth day all the characteristic 
symptoms of pleuropneumonia were developed. Generally 
during the progress of the disease through a herd, fresh cases 
