48 Transactions. — Miscellaneous. 



the body of that animal ; but nothing proves that, if the 

 microscopic organism penetrates into another of the thousands 

 of species in creation, it will not take possession of it and 

 render it diseased. Its virulence, thus strengthened by suc- 

 cessive passages through individuals of that species, might at 

 length attain a condition in which it would be able to attack 

 large animals — man, or certain domestic animals. Intliisway 

 new virulences and contagions may be created. I am greatly 

 inclined to believe that in this manner have aj^peared in the 

 course of ages small-pox, the plague, yellow-fever, &c., and 

 that owing to phenomena of this kind certain great epi- 

 demics appear, such as typhus, just mentioned." 



This mention of the subject of attenuation and vaccination 

 would be incomplete without some notice of M. Pasteur's 

 method of treating hydrophobia, which occurs in the dog as 

 rabies. The natural disease in the dog might take weeks or 

 months to develope. This length of time was practically pro- 

 hibitive to the experimenter. Judging from the symptoms 

 that the virus would be found chiefly in the central nervous sys- 

 tem, he inoculated the brain of rabbits with a portion of the 

 spinal marrow of a dog which had died of rabies, and, passing 

 the virus from rabbit to rabbit, a virus was obtained infinitely 

 more virulent than that procm-ed from the original source — 

 i.e., the diseased dog. By inoculating the \drus directly into 

 the brain-membranes the development or incubation of the 

 disease was shortened. It will give some idea of the time and 

 patience consumed in these investigations if I mention that 

 the stage of incubation in the first rabbit was fifteen days. 

 After twenty to twenty-five passages from rabbit to rabbit the 

 incubation-stage became shortened to eight days. This length 

 of incubation- stage was maintained for a new period of twenty 

 to twenty-five passages, when . the duration of incubation be- 

 came lessened to seven days, which was maintained for a 

 series of ninety new passages of virus from rabbit to rabbit. 

 After this the incubation-period is six days, when the virus 

 attains its maxinnnu intensity and becomes "fixed." By 

 suspending portions of the spinal cord in dry air from one 

 to fourteen days they w^ere found to have lost their virulence 

 in proportion to the time they had been exposed. By very 

 long exposure their virulence became extinct, so that rabbits 

 inoculated with the most attenuated cords — that is, of fourteen 

 days' exposure — were found to be unaffected with the disease. 

 It was thus possible to procure a virus of any desired strength. 

 Dogs inoculated at first with the fourteen-day cord, on the 

 next day with the thirteen-day cord, next day with the 

 twelve-day cord, and so on, were found to bear inoculation 

 with unattenuated virus, and to be fully protected against 

 rabies, whether inoculated with virulent matter or bitten by 



