58o MICRO-ORGANISMS AND DISEASE [chap. 



sterilised by being heated to 60-70° C. for ten or twelve 

 minutes). The dose for subcutaneous must be larger than 

 for intraperitoneal injection ; the dose is at first subfatal, but 

 sufficient to produce distinct illness, then after a week or 

 ten days a second injection is made with a larger dose, then 

 a third, a fourth, a fifth, and a sixth injection, till no reaction 

 at all follows, If ultimately, a fortnight after the last injection, 

 such a prepared animal be tested with a dose of living 

 microbes more than sufficient to kill an unprepared control 

 guinea-pig, it will be found that the prepared animal shows 

 no reaction whatever, and that the living microbe very soon 

 after its injection degenerates and disappears. 



From these and similar experiments as also from experi- 

 ments such as immunisation of guinea-pigs by intraperitoneal 

 repeated injections with diphtheria bacilli, it seems feasible 

 to assume that the immunising or germicidal or antimicrobic 

 potency of the blood-serum in naturally acquired immunity 

 as also in immunity produced by injection of living or dead 

 microbes owes its origin principally or in part to substances 

 derived from the bacterial bodies. 



Now, in the case of the vibrio cholerse or of vibrio Finkler, 

 by cultivating them in solidified blood-serum, which is 

 liquefied by the growth, it will be found that after some 

 weeks' growth at 37" C. a powerful toxin is produced in 

 these cultures, which when used free of the living bacilli (or 

 after sterilisation by heat) affects and kills guinea-pigs pre- 

 viously immunised by dead vibrios against living cultures in 

 the same way and to the same degree as unprepared 

 animals. 



Acquired or artificial immunity against a specific toxin or 

 a specific microbe may be limited to a single tissue or it 

 may involve the whole body, thus Cobbett and Melsome 



