CH. VII] VARIATIONS^ IN PATHOGENICITY 103 



logical characters this organism showed certain resemblances 

 to B. coli. The two organisms could be distinguished from 

 each other most certainly by animal inoculation. Subcutaneous 

 inoculation of the first named into the guineapig gave rise to 

 typical nodular, necrotic, purulent changes in the lymphatic 

 glands, omentum, pancreas, liver, spleen, and lung, an effect 

 which B. coli and its varieties did not produce. 



Again, Shattock (and others, 1907) regards the avian tu- 

 bercle bacillus and the human tubercle bacillus as two distinct 

 species on the ground that, whereas the former when inoculated 

 into guineapigs produces merely a local or a local and glandu- 

 lar disease, the latter produces visceral disease as well. 



Savage (1908-9) has recorded some interesting experiments 

 illustrating the value of animal inoculation in revealing differ- 

 ences in pathogenicity. He found that streptococcus mastitidis, 

 which causes mastitis in the cow, was non-virulent to mice 

 and other rodents but possessed to a marked degree the power 

 to produce mastitis in goats when inoculated into the mammary 

 ducts, and was thereby differentiated from streptococGUS cm- 

 ginosa (isolated from human sore throat)which, though virulent 

 to mice, did not possess the power to produce mastitis in goats. 

 Continuing his experiments with pyogenic streptococci derived 

 from many sources, he found that, although in their cultural 

 properties and their virulence to mice they displayed wide 

 differences, they all resembled each other in their inability to 

 produce mastitis in goats. One streptococcus, for example, 

 isolated from a fatal lymphadenitis in a boy, after it was in- 

 oculated into the teat of a goat survived for seven months as 

 a harmless saprophyte in the milk passages. 



One other example will suffice. We recognise clinically 

 two types of pneumonia, lobar or croupous pneumonia and 

 lobular, catarrhal or broncho-pneumonia. Both types may 

 result from infection of the lung by the pneumococcus. The 

 invading or^nism is apparently identical in the two cases, 

 judged by the ordinary cultural and morphological tests, and 

 the difference in the results produced are therefore attributed 

 to differences in the age and vitality of the patient and the 

 route of infection. 



