244 
Third. For the development of vaccinia it is necessary that the virus 
directly reach a susceptible epithelial surface. It may be placed on such 
a surface or it may be carried there by the blood after having been injected 
into the blood circulation. The disease may also be transferred from 
individual to individual by immediate or intermediate contact, but there 
is no evidence that the virus can be transported by the air as can that of 
smallpox. 
It agrees with smallpox in the similarity of the lesion produced by 
inoculation to the pock and in the fact that both diseases may be produced 
by the virus of variola. 
If material from a smallpox lesion be placed in contact with a suscep- 
tible epithelial surface of man or of the monkey, there develops at the site 
of inoculation a lesion larger, but having the general characteristics of the 
pock, together with constitutional disturbances and an exanthem less 
abundant but otherwise similar to the exanthem of smallpox. Immunity 
to both vaccinia and smallpox follows the disease. All that we know of 
variola inoculata in man is from the old literature. Inoculation of small- | 
pox to confer immunity is no longer practiced in civilized lands. Plehn 
mentions that it is still practiced among the natives in Central Africa. 
We know that the disease so produced is incomparably milder than small- 
pox. The best results were obtained when the inoculation was made 
superficially. The period of incubation is 8 days. ‘There is no doubt 
that the practice of inoculating smallpox to confer immunity would have 
been extensively used and possibly still used were it not for the fact that 
the inoculated individual is capable of transmitting to others the true 
disease. There is no qualitative difference in the virus of variola inocu- 
lata as compared with that of variola vera. The disease differs from 
variola vera in its milder course and shorter period of incubation. There 
are no records of inoculation being made elsewhere than on the skin. ‘The 
lymph nodes become swollen, but there have been no histological examina- 
tions of the skin lesions nor of the internal organs of man. 
We believe that the disease which is produced in monkeys by inoculation 
with smallpox virus most closely corresponds with variola inoculata in 
man and we have so spoken of it. In variola vera the infection is due 
to a virus which can be carried by the air, and infection usually takes 
place without either mediate or intermediate contact. The monkey is 
not susceptible to an air borne-virus. The disease was never transmitted 
from an infected animal to others in the same cage. The monkeys were 
exposed to the disease in the wards and infected material was placed in 
the cages with them. Thinking that the anthropoid apes might prove 
more susceptible, Orang utans were procured from Java and subjected to 
the same conditions. One of the Orang utans was given a blanket from 
a smallpox patient, which it used to cover itself with, but without infec- 
tion ensuing. 
