PREVENTIVE INOCULATION. 123 



under the influence of different agents in Nature — heat, light, chem- 

 icals. When a virus is first obtained from a patient or outside a patient 

 its preceding history, its antecedents, the conditions under which it 

 lived before, are extremely variable. Jenner's method of cultivating 

 the smallpox virus by transferring it from calf to calf secured for that 

 virus uniform conditions of life, and its strength could thus be main- 

 tained unchanged for an indefinite length of time. Pasteur, in the 

 preparation of hydrophobia vaccine, followed the same plan, and found 

 in the successive inoculation from rabbit to rabbit a method of propa- 

 gating the hydrophobia virus in a uniform condition. But attempts 

 made to cultivate in a similar way the comma bacillus by transferring it 

 from animal to animal failed. 



The most susceptible animals for the cholera microbe are Guinea 

 pigs. There are two principal methods of ingrafting upon them the 

 virus: Koch's method of administering it through the mouth and 

 leaving it to develop in the intestines of the animal, and Pfeiffer's of 

 injecting it, not into the intestines, but into the abdominal or peri- 

 toneal cavity, where the intestines are lodged, by introducing there the 

 virus with a hypodermic needle not allowed to penetrate into the intes- 

 tines themselves. But by neither of these methods could the microbe 

 be cultivated in an unbroken series of animals, as it became gradually 

 weakened and soon lost its power of affecting such animals. For the 

 purpose in question, cultivation in the peritoneal cavity had the advan- 

 tage that in a healthy individual the peritoneum is free from other 

 microbes, whereas in the intestines there are always present a large 

 number of micro-organisms which interfere in variable ways with the 

 growth of the particular microbes. 



But when one inoculates the peritoneal cavity of a Guinea pig 

 with a dose of cholera microbes sufficient to cause a fatal dis- 

 ease, it is found, when the animal dies, that the microbes have died 

 also. Thus, the attempt to ingraft the virus from a first animal to a 

 subsequent one is checked at the very beginning. This initial diffi- 

 culty was overcome by merely giving to the first animal a dose larger 

 than was necessary to cause a fatal effect. The animal then suc- 

 cumbs more rapidly, and the microbes have no time to disappear. At 

 the post-mortem examination there is found, in the peritoneal cavity, a 

 small amount of exudate liquid which contains large numbers of those 

 microbes alive. When, however, that exudate is injected into the peri- 

 toneal cavity of a second animal that animal does not succumb to the 

 infection, or even if it succumbs one finds that the microbes have again 

 disappeared in this second animal. By starting with % a still larger 

 initial dose one may have three, perhaps four, successive animals affected 

 by the virus, but it invariably ends by being weakened, and finally 

 dies out. 



