PLEUROPNEUMONIA. 
827 
will occur in a fortnight to three weeks after the first sym- 
ptoms have been noticed in one or two animals; it is not 
uncommon for a month to elapse before the occurrence of a 
second attack ; as the disease goes on the interval between 
the attacks becomes less, so that in a large herd several fresh 
cases may occur in a week. Occasionally long intervals in- 
tervene between the introduction of fresh stock and the de- 
claration of the disease ; one, two, and even three months 
have been known to pass before any symptoms have appeared 
in the animals last imported. A question always arises in 
these cases as to the time and manner of infection ; if it can 
be reasonably assumed that the disease may remain in the 
system for three months without any external manifestations 
of its presence, the difficulty ceases ; but as all the most 
direct evidence is in favour of the opinion that the average 
time of incubation is about twenty days, such an assumption 
cannot be sustained, and we are forced to conclude either 
that the animal has been infected by indirect conveyance of 
the virus, or that pleuropneumonia is regulated by laws 
which are not clearly understood. 
It cannot be doubted that the amount of the poison re- 
quired to induce the disease in the system of a healthy animal 
is in inverse proportion to the state of susceptibility, and 
thus a non-susceptible animal may resist the action of the 
virus which it receives w 7 hile associating w 7 ith diseased cattle 
for several w 7 eeks, w 7 hile another with a high degree of suscep- 
tibility may be infected immediately, the period of incubation 
would most probably be exactly the same in both instances 
reckoning from the time when infection really took place; 
but in the case of one the disease may appear to have re- 
mained latent for three months, and in the other for less than 
the same number of w 7 eeks, in consequence of the different 
degrees of resistant pow 7 er in the system. 
Nature , Symptoms , and Lesions of Pleuropneumonia. 
Inflammation of the substance of the lungs, and the pleural 
membrane which is reflected over them, is a disease common 
to man and the lower animals, and, long before the cattle 
epizootic was known at all, the term pleuropneumouia was 
employed to designate this ordinary inflammation of the 
lungs and its results. When the disease w 7 as first recognised 
on the Continent, the expression which had hitherto sufficed 
to indicate a common- disease of lungs and pleura was used 
in application to the epizootic, only being qualified by the 
terms exudative or contagious, w hich referred to its special 
