II. 



On Pasteur's Method of Inoculation and its 

 Hypothetical Explanation. 



" T N Nature's infinite book of Secrecy," the bacillus and kindred orga- 

 i nisms form an interesting, and of late a much-read, page. Their 

 very minuteness, and the difficulty of studying them, lends an 

 additional fascination ; and they rise into painful importance in the 

 light of the modern view that they are the cause of disease. 



Since splenic fever was first attributed to the bacillus by the two' 

 French observers, MM. Davainne and Rayer, in 1861, one disease 

 after another has followed, until finally the cause of influenza has- 

 revealed itself under the microscope of the investigator. To give a 

 list of diseases now attributed to it, would be to compile a page 

 from a medical treatise. 



All this renders the bacillus of paramount interest, yet a still 

 greater interest attaches to the extraordinary pathways to health 

 pointed out by the brilliant researches of Pasteur. I allude to his- 

 well-known system of inoculation in order to confer immunity 

 from disease. 



The foundation of Pasteur's process is the cultivation of the 

 microbe — bacillus, micrococcus or bacterium — outside the animal body 

 in a suitable medium. Various substances — meat broth, sugar and 

 peptone, Liebig's extract, &c, — are used in which to grow the microbes. 

 These liquids, or solids, must be sterilised — that is to say, heated 

 until all germs which may have existed in them have been destroyed 

 — and afterwards protected from atmospheric germs by plugs of 

 cotton wool. With these precautions it is found possible to obtain 

 pure cultivations of any microbe which may be sown in the medium. 



Into a tube or flask, then, containing this medium a drop of blood, 

 or fragment of tissue, from a diseased animal is introduced. The 

 microbe existing in it at once increases in numbers, and soon renders 

 the medium turbid. A drop of this first cultivation introduced under 

 the skin of a healthy animal reproduces the original disease with 

 which it was associated. To obtain a second cultivation, a drop 

 from the first is placed in another portion of the medium, where 

 the microbe increases as it did in the first. Proceeding in this 

 way, a succession of cultivations may be obtained. Now Pasteur 



