EXPERIMENTS ON PLEURO-PNEUMONIA. 
353 
indeed, been alleged by some authorities that actual lung 
disease can be generated by the insertion under the skin of 
bits of diseased lung; but this inference, which if it were well 
established would be of great importence, can be shown to be 
mistaken. The observations quoted in support of it are too 
good to be true. In most instances, the time which intervened 
between the inoculation and the appearance of lung disease 
was far too short; for we have evidence from the pathological 
inquiries of Professor Yeo, as well as from other sources, that 
the development of the disease in the lungs requires a very 
long time, and usually produces no obvious symptoms at all 
until it breaks out in the acute form in which it is ordinarily 
recognised. Consequently, the appearance of symptoms 
within a week or two after inoculation could not reasonably 
be referred to the operation as their cause; so that we need 
not hesitate to conclude that the animals in question had been 
previously infected by other means. 
Another statement that has been made with reference to 
the mode of action of inoculation is equally unfounded, viz. : 
that although inoculation never produces actual pleuro-pneu- 
monia, yet, that it gives rise, at the place where the morbid 
material is introduced beneath the skin, to a local disease 
which is of the same kind as the real disease of the lungs, and 
that consequently the effect of the inoculation is to produce 
a sort of pleuro-pneumonia of the skin ! Now it is quite true 
that there is a great resemblance between them—a likeness 
sufficiently striking to have impressed some very well informed 
persons—but very little stress ought to be placed on it. All 
inflammatory exudations, whether specific or not, are very 
like each other as regards their chemical and anatomical 
characteristics; so much so that it is not possible to dis¬ 
tinguish them from each other excepting by their disease- 
producing properties. In other words, the only way in which 
it would be possible to prove that any diseased material derived 
from the skin of the inoculated animal was pleuro-pneumonic 
would be by showing experimentally that when introduced 
into another animal it produced pleuro-pneumonia. If this 
proofwere given we should have a rightto concludefromanalogy 
with similar cases, that in all probability immunity would be 
conferred on the infected animal; but in the absence of such 
proof, the only way in which the protective power of inoculation 
can be settled for practical purposes, is by observing whether 
inoculated animals can get pleuro-pneumonia by exposure. 
The experiments which had previously been made for this 
purpose were unquestionably in favour of the protective power 
of inoculation. The inquiries of the French Commission, 
