552 MICRO-ORGANISMS AND DISEASE [chap. 



gradations in both classes of microbes depending (a) on the 

 initial character (or virulence), some species acting in smaller 

 doses than others, {b) on the quantity injected, {c) on the 

 greater or lesser reactivity, smaller or greater resistance of the 

 tissue in particular or the animal in general. 



This brings us now to the consideration of the subject 

 of what constitutes this resistance or immunity, what is the 

 cause of this natural resistance or spontaneous insuscepti- 

 bility, observed at the outset. 



It must be clear from what has been just stated that the 

 greater or lesser immunity or resistance of the tissue or of 

 the animal are relative quantities. Under spontaneous or 

 natural resistance is meant the natural capability of a tissue 

 to withstand the growth and multiplication of a microbe and 

 to destroy or kill the latter. And this spontaneous immunity 

 must be distinguished from acquired or secondary immunity. 

 We have shown that while one tissue is capable of destroying 

 the microbes brought in contact with it, another tissue of the 

 same animal does not achieve this result, or while an animal 

 in one condition is found resistant it is found susceptible 

 under another condition, or again while an animal is sus- 

 ceptible towards a microbe in an unaltered virulent condition 

 it is possessed of resistance against an altered or attenuated con- 

 dition of the same microbe, and lastly, while a particular tissue 

 or the animal is insusceptible and found resistant against 

 a small dose it is found susceptible against a large dose. We 

 will mention here a few examples illustrating these points : — 



{a) The cholera vibrio, the vibrio of Finkler, the bacillus 

 prodigiosus, the bacillus coli, and many other microbes when 

 injected into the peritoneal cavity of a normal guinea-pig 

 in sufficient doses live and grow well and produce acute peri- 

 tonitis and death, while when introduced in the same amount 

 into the subcutaneous tissue or into the blood they soon 



