Pasteur's Treatment 413 



the pancreas, and the nervous system contain the infection, 

 and are more appropriate for experimental purposes than 

 the saliva, which is invariably contaminated with accidental 

 pathogenic bacteria. 



The introduction of a fragment of the medulla oblongata 

 of a dog dead of rabies, beneath the dura mater of a rabbit, 

 causes the development of typical rabies in the rabbit in 

 about six days. It is only by such an inoculation that a posi- 

 tive diagnosis of the disease can be made. The operation 

 must be performed with the greatest care in order to avoid 

 septic infection with meningitis. The technic is simple, a 

 small trephine for opening the rabbit's skull being obtainable 

 from the dealers, though in its absence the thin bone of the 

 cranial cavity may be cut with a heavy scalpel. The material 

 to be inoculated should be crushed to a fine pulp in sterile 

 physiologic salt solution, and introduced beneath the dura 

 with a hypodermic syringe. The tissue of the medulla of a 

 rabid rabbit introduced beneath the dura mater of a second 

 rabbit produces a more violent form of the disease in a shorter 

 time, and by frequently repeated implantations Pasteur 

 found that an extremely virulent material could be ob- 

 tained. 



Pasteur observed that the virulence of the poison was less 

 in animals that had been dead for some time than in those 

 just killed, and by experiment found that when the ner- 

 vous system of an infected rabbit was dried in a sterile at- 

 mosphere its virulence attenuated in proportion to the 

 length of time it was kept. A method of attenuating the 

 virulence was thus suggested to Pasteur, and the idea of 

 using it as a protective vaccination soon followed. After 

 careful experimentation he found that by inoculating a 

 dog with much attenuated, then with less attenuated, then 

 with moderately strong, and finally with a strong, virus, it 

 developed an immunity that enabled it to resist infection 

 with an amount of virulent material that would certainly 

 kill an unprotected dog. 



It is remarkable that this theory, based upon limited accu- 

 rate biologic knowledge, and upon experience with very few 

 bacteria, should find absolute confirmation as our knowledge 

 of immunity, toxins, and antitoxins progressed. Pasteur 

 introduced the unknown poison-producers, attenuated by 

 drying, and capable of generating only a little poison, accus- 



