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Concluding Report on the Experiments at the 
becoming the seat of diffuse cellular infiltration of the same 
kind as that which has already been described. In the ordinary 
case, a common result of tail inoculation is that part of the 
organ separates by necrosis, an event which is often not attended 
with any serious disturbance of the animal's health (see Appen- 
dix III.). 
There are two reasons why inoculation, as practised in the 
above instances, is necessarily attended with danger. One of 
these is that the liquid used, however carefully it is collected, 
not only contains the virus of pleuro-pneumonia, but possesses 
morbific properties of another kind, which are due to its being 
the product of an unhealthy, or, as it is often called, an infective 
inflammation — properties which would have manifested them- 
selves if, instead of the juice of a pleuro-pneumonia lung, we 
had used any other exudation-liquid of a similarly infective 
character. A second source of danger is, that the living tissue 
which serves as the channel of introduction is one which we 
know to be particularly susceptible to infective influences of this 
kind. By previous experiments, relating to another inquiry, 
we had found that this second danger could be avoided by 
infusing the morbific liquid directly into the circulation. We 
therefore resolved to adopt this plan, feeling sure that, if the 
pleuro-pneumonic virus possessed any protective power at all, 
that power would be exercised to the greatest advantage if the 
liquid were mixed with the circulating blood ; for in this way 
it would necessarily come into contact, not merely with any 
particular part, but with every organ in the body. Another 
advantage which the method of infusion into the blood-stream 
offered was, that from fifty to a hundred times the quantity of 
liquid could be introduced at once, and thus the chance of 
infection be vastly increased. 
Fourteen animals were thus inoculated (see Appendix IV.). 
The instrument employed was a syringe, capable of containing 
two drachms of virus, and furnished with a slender, sharp- 
pointed steel tube. The point was made to enter the principal 
vein by which blood returns from the back of the ear, and was 
usually secured by a ligature. The syringe was then 'slowly 
discharged, the greatest care being taken to avoid contact with 
the cellular tissue. It was often unnecessary to divide the skin. 
The whole operation was conducted without any appreciable 
suffering to the animal. 
In the first batch of eight animals the operation was, in all 
but one, performed twice in each case, at an interval of several 
weeks, with a view to greater certainty of result. With the 
exception of a small prominence which marked the seat of the 
inoculation, and subsided in a few days, it was in general 
