650 APPENDIX. 



to count on immunity for more than two years, and 

 whenever we are liable to exposure it is well to be 

 vaccinated. If it was unnecessary it will not be suc- 

 cessful, while if it is successful we have reason to 

 believe we were liable to at least a mild smallpox 

 infection. 



Protective Substances Present in the Serum of Animals 

 After Successful Vaccination. It has been repeatedly 

 shown that the blood-serum of a calf several weeks 

 after successful vaccination possesses feeble protective 

 properties, so that the injection of one to two litres of 

 it into a susceptible calf would prevent a successful 

 vaccination. A further and more convincing fact has 

 been demonstrated in the laboratory by Huddleston 

 namely, that when equal parts of a serum taken from 

 a calf, two weeks after successful vaccination, and of 

 an active vaccine are mixed together and inoculated 

 in a susceptible calf the vaccine fails "to take." 

 The serum of an unvaccinated calf has no deleterious 

 effect whatever when mixed with the vaccine. Serum 

 taken even several years after vaccination, if the case 

 is still immune, will inhibit very distinctly the action 

 of fresh vaccine virus. 



The Form of Vaccine Virus Used. Vaccination is 

 now usually performed with calf virus, as this is easier 

 to obtain, is just as reliable, and practically eliminates 

 the slight possibility of the transferrence of syphilis 

 which existed in human vaccine. With active virus 

 a portion of skin only one- sixteenth of an inch in 

 diameter is scratched with the needle and the virus 

 rubbed in. If preferred it may be inserted by a 

 puncture. The vaccine is now usually mixed with 

 glycerin and water and placed in capillary tubes. So 



