444 PLAGUE. 



From the facts stated with regard to the powers of compara- 

 tively rapid multiplication of the bacillus, its wide dissemination 

 by affected rats, human excreta, etc., it may be understood how 

 extensively the soil and dwellings may become infected, and 

 how difficult it may be to arrest the ravages of the disease. 

 How important a part such infection of a locality plays is strik- 

 ingly shown by the rapid fall in the number of cases when the 

 people go into tents. 



Toxins, Immunity, etc. As is the case with most organisms 

 which extensively invade the tissues, the toxins in plague cultures 

 are chiefly contained in the bodies of the bacteria. Injection of 

 dead cultures in animals produces distinctly toxic effects ; post- 

 mortem, haemorrhage in the mucous membrane of the stomach, 

 areas of necrosis in the liver and at the site of inoculation, may 

 be present. The toxic substances are comparatively resistant to 

 heat, being unaffected by an exposure to 65 C. for an hour. By 

 the injection of dead cultures in suitable doses a certain degree 

 of immunity against the living virulent bacilli is obtained, and, 

 as first shown by Yersin, Calmette, and Borrel, the serum of 

 such immunised animals confers a degree of protection on 

 smaller animals, such as mice. On these facts the principles of 

 preventive inoculation and serum treatment, presently to be 

 described, depend. It may also be mentioned that the filtrate 

 of a plague culture possesses a very slight toxic action, and the 

 Indian Plague Commission found that such a filtrate has prac- 

 tically no effect in the direction of conferring immunity. 



I . Preventive Inoculation Haffkine[s Method. To prepare 

 the preventive fluid, cultures are made in flasks of bouillon with 

 drops of oil on the surface (in India Haff kine employed a medium 

 prepared by digesting goats' flesh with hydrochloric acid at 

 140 C. and afterwards neutralising with caustic soda). In such 

 cultures stalactite growths (vide snpra) form, and the flasks are 

 shaken every few days so as to break up the stalactites and 

 induce fresh crops. The flasks are kept at a temperature of 

 about 25 C., and growth is allowed to proceed for about six 

 weeks. At the end of this time sterilisation is effected by expos- 

 ing the contents of the flasks to 65 C. for an hour ; thereafter 

 carbolic acid is added in the proportion of .5 per cent. The 

 contents are well shaken to diffuse thoroughly the sediment in 

 the fluid and are then distributed in small sterilised bottles for 



