CHAPTER XIV. 
THE USE OF ANIMALS FOR DIAGNOSTIC AND TEST 
PURPOSES. 
SuITABLE animals are necessarily employed for many 
bacteriological purposes. Thus they may be used as a 
soil for bacterial growth, when, as in the case of tubercle 
bacilli, the bacteria will not develop in the dead culture 
media. For this reason material suspected to contain 
tubercle bacilli is injected into rabbits or guinea-pigs, 
with the knowledge that, if present, although in too 
small numbers to be detected by microscopical or cul- 
ture methods, they will develop their lesions in the 
animal’s bodies, and thus reveal themselves. The same 
may be true of glanders and anthrax bacilli and of 
other bacteria. Again, animals are used to test the 
virulence of organisms, where, as in the case of diph- 
theria, we have very virulent, attenuated, and non- 
virulent bacilli of, so far as we know, identical cultural 
characteristics. Here the injection of a susceptible 
animal, such as the guinea-pig, is the only way that we 
can differentiate between those capable of producing 
diseases from those that are harmless. Still another 
use of animals is to differentiate between two virulent 
organisms, which, though entirely different in their 
specific disease-poisons, are yet so closely allied mor- 
phologically and in culture characteristics that they 
cannot always be separated except by studying their 
